Question about an unrelated vehicle that has an issue. Need input.

littleblazer

Gold Supporter
#1
Morning everybody. So as some of you may know I am allowed to drive another vehicle besides the trailblazer. As some of you may know that would be the corvette. It's an 84 and I just wrapped up some work on it last week or so. The issue I'm having is that it runs great. Like perfect, until about 2500 rpms and then it breaks up. But you can power through it and it's only when you're burying the pedal in the carpet. Even stranger is it only does it sometimes.

The story: so after I broke in the new cam we set the timing. The engine coolant sensor failed, it used the old school knock sensor like plug and if you wiggled it it would work so I just replaced it with a unit that uses a more traditional 2 pin plug. Initially you could smell that the car was running lean, factory injectors are marginal at best foe the factory power then adding anything on top of it showed that. Fuel pump also wouldn't hold pressure under load. Okay new pump and injectors and the car runs mint. Punch it out 1-2-3 It was good for 2 or 3 solid pulls up to 5k. Then it started again right at 2500. Alright let's do a pcm relearn. No go. Bump the fuel pressure back up. Better for a bit then bad again.

So two nights ago I'm getting really frustrated and I'm looking under the hood. You can guess what I saw, lightning bolts everywhere. My guess is the plug wires got pretty hot during the cam break in. The manifolds were cherry red but holding a car at 2-2500 rpms for half an hour will do that. Alright so I put new plug wires in (they were 12 years old and pretty stiff anyway so it was a good measure.) Then went for a drive last night. Punched it out and it starts to smoke the tires in second. Awesome I thought. And the next time we took off it broke up.

Observations: it seems to get worse as I drive it but you can power through the break up. It doesn't feel like fuel starvation as it bucks, it wants to go but it's like it's dropping ignition. You can back down to 3/4 throttle and break through it at about 3200 rpm and then it's good up to around 4200. Then it starts again but not as bad. Under light loads it does it slightly in that band but what's weird is pulling back on the gas and hitting it again, like to let fuel flow build up again, doesn't help. Sometimes it does but usually it doesn't. And sometimes it doesn't do it at all.

Things checked: so far fuel pressure is good, new plugs, not leaning out.

Other things to check: I want to look at the throttle position sensor, the coil and maybe revisit fuel. I also want to pull the plugs, I'm wondering if I could have roasted them breaking in the cam as it does run to 230 before the e fan kicks on. Also I had checked the tps a few months back but again, the heat isn't good. I know that they can cause issues as well.

It runs too good everywhere else to be a vacuum leak I would think. I hosed it with starter fluid with no perceivable change in how it ran, idle vacuum is something like 19 inches.

I'm asking because I'm at the point where I'm not that familiar with the system. It's a hei system with the coil on the distributer and it has electronic spark advance. I really only know about the mechanical systems on boats so if it is anything on the computer side I don't know exactly how to test it. The car still pulls hard to the point it starts breaking up but even in the middle of it it still wants to go, it pulls moderately hard in it just not as much. Any input would be appreciated as I'm sure a lot of you guys have some. Either way I will get to the bottom of this but I want to not throw parts at it as all the problem spots have been addressed and now I'll be changing things I'm unsure of. Thanks in advance.
 

AWD V8

Well-Known Member
#3
I first encountered this same problem while working at a VW dealership in the '80's. Brand spanking new car right off the car carrier. Would rev nice and clean until EXACTLY 2500 RPM, then it would misfire badly, backfire. Short story, after many hours of diagnosis on a new type engine that we did not have info on yet, it turned out to be the ignition module. I've since encountered this on 2 GMs and a few I've diagnosed online.

If you have a behavior that happens at exactly the same RPM, even if it's happening randomly, it is an electronic bug. Something with "logic" is failing at a particular frequency, or duty cycle.

Most recent my son's Mustang did this. His old distributor took a dump when the pickup came loose and got chewed up by the reluctor wheel. I bought a brand new, respected brand name distributor. I installed it and it started up fine, throttle response was great, but when it hit something like 2200 RPM the thing started misfiring like crazy. I quickly thought distributor module when I saw the tach twitching when this was occurring. I had the dealer give me a new module and it ran perfect.

Replace the ignition module in the distributor and I think you will be pleased.
 

christo829

Well-Known Member
#4
Going to do the "me too!" routine. Had a 78 Camaro that did the same thing around 2300 rpm when hot. That distributor mounted coil was the culprit. Apparently it would start to arc once it got hot, and the demand "sweet spot" was about 2300.

Wouldn't hurt to recheck the plugs anyway, but I'm on the coil bandwagon.

Good Luck!

Chris
 
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littleblazer

littleblazer

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#5
So I picked up an ignition module just to have it. Went to check the box and found that someone did a swap and returned their old delco. I was pretty steamed. After I exchanged it I pulled the top off the distributer and checked the coil out with my meter. It tested good but it is the factory part (mould numbers only no part number). So I may do that but I lost steam and just left it. Tomorrow I'll pull the plugs. I'm hoping they're at .055 and then it's an easy fix. I actually suspected the coil after the fuel but the ignition module makes sense as well. Both got roasting hot so I guess that's why it happened after the fact.
 
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littleblazer

littleblazer

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#6
Update: pulled the plugs they looked mint. Pulled the ignition control module, pretty crusty looking so I swapped it out. No change. If anything it may be doing it a little worse but it's also 90 not 80 outside. I'm going to test the throttle position sensor and look into a coil.
 

AWD V8

Well-Known Member
#7
The pickup coil moves on the advance plate. When the wire on the pick breaks internally from the movement over the years, the ignition will cut out. I've replaced the pick ups in these distributors to cure this.

What you can do to confirm that it is the ignition cutting out, is, first confirm that you can duplicate the symptoms without driving the truck. Just rev it to see if it cuts out at any particular rpm. Now, hook a timing light to any plug wire. Start the engine, timing light should be flashing. Slowly increase the RPM and watch the flashing, it should continue, even if the engine begins misfiring. If the flashing stops or is intermittant then you know that the ignition is cutting out. I did this on my son's Mustang.
 
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littleblazer

littleblazer

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#8
The pickup coil moves on the advance plate. When the wire on the pick breaks internally from the movement over the years, the ignition will cut out. I've replaced the pick ups in these distributors to cure this.

What you can do to confirm that it is the ignition cutting out, is, first confirm that you can duplicate the symptoms without driving the truck. Just rev it to see if it cuts out at any particular rpm. Now, hook a timing light to any plug wire. Start the engine, timing light should be flashing. Slowly increase the RPM and watch the flashing, it should continue, even if the engine begins misfiring. If the flashing stops or is intermittant then you know that the ignition is cutting out. I did this on my son's Mustang.
It doesn't appear to cut out when you rev it in neutral. Only with a load does it break up.

Update: car was running worse and worse as I drove it. I swapped the coil out and it ran significantly better but still does it. Not as much or prominent though. I'm ready to torch the damn thing. I adjusted the throttle position sensor and it ran better along with the new coil. At this point I'm going to swap the cap and rotor just so it has a total tune up...
 
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littleblazer

littleblazer

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#9
Update: new cap and rotor helped part throttle performance. You can bring it up into that range now under part throttle and it feels good but if you hammer it it does it still. But not all the time now. It seems sporadic now.
 
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littleblazer

littleblazer

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#10
Update 2: I was able to duplicate the results in the driveway with no load. There is no visible break up of the timing light flashes. This thing is killing me now. I replaced the o2 sensor as per the service manual (there was lots of rtv that more than likely got burnt off, the sensor was coated and is 15 years old anyway.) As I had one and planned to anyway. The only other source to check is the fuel filter but I replaced that about 500 miles ago so there is no possible way it could be that. I may recheck the plugs again in more detail but it runs too good up to that point for it to be it. I'd rather not keep throwing money at it at this point.

Anyone have any other ideas.

Just to list what's been done, wires, coil, cap, rotor, ignition control module, oxygen sensor. Checked for vacuum leaks with starting fluid to no avail.
 
#11
Did you check fuel pressure and pump above idle? I remember on my old Blazer that had a dying pump where it was fine up to a certain RPM but was starved above a certain RPM as the pump couldn't supply enough fuel. Maybe also check for bad gas?
 
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littleblazer

littleblazer

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#12
I actually upgraded the injectors and fuel pump because stock was marginal well... for the stock setup. Pressure remains good under load. I actually thought that since it is the crossfire system, is it possible that the throttle bodies are a little out of sync? I can't imagine the little that they're off would cause an issue like this but I may check the balance again with a vacuum gauge on each one to see.
 

Tiggerr

Well-Known Member
#13
I actually upgraded the injectors and fuel pump because stock was marginal well... for the stock setup. Pressure remains good under load. I actually thought that since it is the crossfire system, is it possible that the throttle bodies are a little out of sync? I can't imagine the little that they're off would cause an issue like this but I may check the balance again with a vacuum gauge on each one to see.
Sounds like an ignition issue to me... maybe a pickup coil? A little slop in the dist shaft even?
That said... there was a reason crossfire injection went away after 3 yrs lol... I remember some people pulling their hair out over that shit back in the day when those were much newer....
Most people I knew shit-canned that setup and went 4bbl on a good intake...or later on swapped TPI on them...
 
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littleblazer

littleblazer

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#14
Sounds like an ignition issue to me... maybe a pickup coil? A little slop in the dist shaft even?
That said... there was a reason crossfire injection went away after 3 yrs lol... I remember some people pulling their hair out over that shit back in the day when those were much newer....
Most people I knew shit-canned that setup and went 4bbl on a good intake...or later on swapped TPI on them...
See, I've lost more hair over people's tpi than I ever did with this lol. There really isn't much to the system which is what I like. While tpi is better both are so antiquated at this point it's hard with either. Like my buddy has been chasing a maf issue on his tojan for 2 years now. He rewired the damn thing replaced the ecm and all components and it still falls on it's face. I'm coming to accept that it's just 80s gm.
 
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littleblazer

littleblazer

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#15
1 3 5 7 (both images are upside down for whatever reason...)
20170724_180943.jpg
8 6 4 2
20170724_184126.jpg
 
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littleblazer

littleblazer

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#16
So I think I've fixed it. Anyone care to take a guess what it ended up being?
 
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littleblazer

littleblazer

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#18
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littleblazer

littleblazer

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#20
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littleblazer

littleblazer

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#22
Alright. It still does it a little. Not as bad but enough that you kind of notice it. Running back to back gets rid of it. I had heard of an issue with delco #2 plugs the iridium ones, causing weak spark. So I put in a set of regular copper delco and it pretty much went away unless you idled for a while. So then I dialed down the fuel pressure effectively lowering the amount delivered and that mostly fixed that. It's 85% solved at the moment I just need to fiddle with fuel a little more and I think it'll be good.
 
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littleblazer

littleblazer

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#24
All that for the wrong plugs? Rough! Glad you got it mostly figured out.
Not so much the wrong plugs, they're the right ones just iridium instead of copper. The iridium is supposed to be better but it's not the case here as there is a slight defect in the #2 plugs vs the r45st or whatever they are.
 
#25
Just like the 4.2 just likes the iridiums, your old Vette probably likes the old stuff. Different eras.
 
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littleblazer

littleblazer

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#26
Perhaps. Aside from the breakup I really didn't notice any difference in how it behaved tbh. I'll probably read the new plugs this week and see how the fueling is. I don't like that it is starting to do it a little again.
 
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littleblazer

littleblazer

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#27
So I haven't driven the car since that last post. Today I balanced the throttle bodies and let me just say that car has never launched like that. But it still does it. The vast way to describe it is like it's hitting a rev limiter or ignition is cutting. When it does it it kind of just stops at that point. Being honest I don't know if it's done this before the work. I never really punched it out since it was... well... lame. It doesn't do it all the time it's about 50/50. But when it goes it goes. I'm almost thinking an ecm issue but I want to explore the pick up in the distributer too. I haven't driven it because it's been annoying me.
 
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littleblazer

littleblazer

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#28
I'm going to run a compression test this weekend to rule out a cracked head. The 624s tend to do that and they did stay quite hot during cam break in. I tried to get a video of the issue. Needless to say it really isn't prevalent here it's actually almost perfect but a few moments before I let up in second you can hear a slight change in the exhaust note. When that change occurs you can feel it but it also feels right when that happens. The last little bit comes out. Yesterday driving around it behaved like poop. Did it all the time at part and full throttle. Scratching my head but the car ran warmer then when it doesn't do it. It really doesn't seem to follow a pattern and I'm about to set a lighter to the thing and be done with it. Just updating the saga.

 
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littleblazer

littleblazer

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#29
So I may have accidentally figured out the problem... It was a combination of two things and my original original assumption was correct. I had a small vacuum leak. The first was to the egr control solenoid, the little hose off the throttle body that goes to the valve that controls the egr valve itself. That was my fault as I cracked the hose during reassembly and just shortened it to make it work. Long story short my vacuum pump could generate zero vacuum on it. The second was the damn intake manifold. This is the third problem I have had with it. The outer bolts for the throttle body go down into the wall of the manifold and the damn hole wasn't drilled and tapped deep enough. I figured I would re snug the bolts and all were still tight but I noticed something. With the socket on the bolt the head moved. It looked like it was down all the way but wasn't. Two washers fixed that and I took it on a long drive (30 miles). Not even a hiccup. The real test is letting dad drive as it always seems to behave better with me behind the wheel.

Testing for leaks with the egr plugged showed slight leaks on both throttle body bases. It took a lot of starter fluid to catch it though. I never needed to spray for 5 or 10 seconds to see a leak but when it's got to get sucked past an inch of gasket or so I guess you need a bit to see it. Fingers crossed I finally got it because the car is an animal the way it currently runs.
 
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littleblazer

littleblazer

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#30
Never mind...
 

MRRSM

Lifetime VIP Supporter
#31
I was thinking about your engine spark break up issues and I remembered having a similar problem with a GM 6/60 V6 relating to an excessive amount of heat affecting the insulation on the Spark Plug Wires. Re-looming was problematic due to the confined spaces around the engine ...so after a bit of research... I found these:

https://www.amazon.com/Titanium-Spa...coding=UTF8&psc=1&refRID=2VHKSXNZ5MYSWXRYJZKR

After slipping these over and the SPWs adjacent the headers of the engine... No More Breaks in Spark or power lags at higher RPM. It is important to know that right along with raising the RPM ...the higher compression conditions will change the amount of resistance that a Spark must overcome to jump the gaps when exposed to the denser atmospheric High Compression events created for each ignition and subsequent power stroke. So knowing whether or not the Quench inside the combustion chambers has been altered either by using a Thicker ...or a Thinner Head Gasket to increase or decrease compression would directly affect the need to change the Spark Plug Gap to fit the situation. This is an area worth investigating with the GM V8 Hot Rod aficionados for better data as well:

http://www.hotrod.com/articles/ideal-quench-height/
 
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Locksmith

Silver Supporter
#32
As I am an old guy, let me add that on most GM's that had HEI ignition, and whenever there were stalling or no starts, because of ionization under the distributor cap, the first thing to check is to take the distributor rotor off and look under the center of the cap for a tiny pin hole! It is a sign that the spark has grounded itself to where the centrifugal advance plate is. FYI.
 

Locksmith

Silver Supporter
#34
...and with that same idea in mind... Look at the low price on this item:

https://www.amazon.com/gp/product/B...fl_title_12?ie=UTF8&psc=1&smid=A34668PDYO45KB
...and with that same idea in mind... Look at the low price on this item:

https://www.amazon.com/gp/product/B...fl_title_12?ie=UTF8&psc=1&smid=A34668PDYO45KB
Ionization under the distributor cap, would create "carbon tracks" and your spark would short out in the wrong places, and the old hot rodders would drill 1/2 inch holes in the sides of the caps to let the ions vent, but it made the ignition point vulnerable to water!
 
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littleblazer

littleblazer

Gold Supporter
#35
I was thinking about your engine spark break up issues and I remembered having a similar problem with a GM 6/60 V6 relating to an excessive amount of heat affecting the insulation on the Spark Plug Wires. Re-looming was problematic due to the confined spaces around the engine ...so after a bit of research... I found these:

https://www.amazon.com/Titanium-Spa...coding=UTF8&psc=1&refRID=2VHKSXNZ5MYSWXRYJZKR

After slipping these over and the SPWs adjacent the headers of the engine... No More Breaks in Spark or power lags at higher RPM. It is important to know that right along with raising the RPM ...the higher compression conditions will change the amount of resistance that a Spark must overcome to jump the gaps when exposed to the denser atmospheric High Compression events created for each ignition and subsequent power stroke. So knowing whether or not the Quench inside the combustion chambers has been altered either by using a Thicker ...or a Thinner Head Gasket to increase or decrease compression would directly affect the need to change the Spark Plug Gap to fit the situation. This is an area worth investigating with the GM V8 Hot Rod aficionados for better data as well:

http://www.hotrod.com/articles/ideal-quench-height/
As I am an old guy, let me add that on most GM's that had HEI ignition, and whenever there were stalling or no starts, because of ionization under the distributor cap, the first thing to check is to take the distributor rotor off and look under the center of the cap for a tiny pin hole! It is a sign that the spark has grounded itself to where the centrifugal advance plate is. FYI.
...and with that same idea in mind... Look at the low price on this item:

https://www.amazon.com/gp/product/B...fl_title_12?ie=UTF8&psc=1&smid=A34668PDYO45KB
Ionization under the distributor cap, would create "carbon tracks" and your spark would short out in the wrong places, and the old hot rodders would drill 1/2 inch holes in the sides of the caps to let the ions vent, but it made the ignition point vulnerable to water!
Thanks for the input guys! I've come to the conclusion that it's throttle related as if you just stab it it behaves fine. It's part throttle that acts up. As much as I'd like to drop in a new distributor the one on there is fully electronic so I can't really use one with vacuum advance. I have a new throttle position sensor as messing with it at idle create a similar issue where it seems like it may be the problem. It tests fine with a meter but it could have a problem that only the computer may see. I'll pull the cap and rotor again and check everything out. Here's a video so you can actually hear what it's doing. If you listen you can hear it get burbley if you will around 2900.
 

Locksmith

Silver Supporter
#36
I got a chubby listening to the music! I didn't hear any burps or something out of the ordinary. I cleaned out the throttle body of my 06 Civic by turning ignition on and used a brick on the accelerator being careful not to move the plate, and had to take it to the dealer to have the computer synch the tb because it surged by itself in reverse gear. Back in the day it would be the carburetor accelerator pump! Effin computers!
 
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littleblazer

littleblazer

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#37
I got a chubby listening to the music! I didn't hear any burps or something out of the ordinary. I cleaned out the throttle body of my 06 Civic by turning ignition on and used a brick on the accelerator being careful not to move the plate, and had to take it to the dealer to have the computer synch the tb because it surged by itself in reverse gear. Back in the day it would be the carburetor accelerator pump! Effin computers!
On the first one the droning like sound maybe? There is a slight tone change around 2800. That is it. Here's another you may be able to hear better. Its hard to pick up in the car as it just sounds like a drone. The sync is done mechanically via a vacuum gauge between the two throttle bodies then you just set the voltage on the sensor with a volt meter to .525 volts. It's before all the real special computers.
 

Locksmith

Silver Supporter
#38
My 69 Camaro with alumnacoat headers used to "ring" at a certain rpm! Lived with it and loved the sound.
 
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littleblazer

littleblazer

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#39
My 69 Camaro with alumnacoat headers used to "ring" at a certain rpm! Lived with it and loved the sound.
My problem is it didn't do it before the cam and intake. So something is up. It's a lot faster though.
 

Locksmith

Silver Supporter
#40
A Chevy V-8 18436572 firing order, you always had to be careful not to run the plug wires side by side, especially cylinders 5 & 7 because a voltage surge could be induced into the other cylinder causing misfire's! Just make sure the 5 & 7 are separated and cross somewhere along the parallel wire looms.
 

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