intermittant high idle still

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
ok still have intermittant high idle problem. starts when I fill up with fuel and then immediatly drive on highway come to stop at next town 13km's idle up at about 950 stays up at 950 idle for 80 to 120km's then goes back to 650 and stays that way for rest of tankfull but if i fill up and go home and just park it next day can start it up and it will idle normally. It has been doing this for 2-3 years. note no codes tb is clean in fact I have two and both do the same truck runs great other than that. any clues to what is going on? truck has 125000km's all maintenance up to date.:confused:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Is your vacuum hose from the fuel pressure regulator to the intake manifold resonator on tightly? If it's idling properly, does it jump to 900 RPM when you pull that hose off? Ever get any SES light codes?
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
the roadie said:
Is your vacuum hose from the fuel pressure regulator to the intake manifold resonator on tightly? If it's idling properly, does it jump to 900 RPM when you pull that hose off? Ever get any SES light codes?

everything is connected and seems tight will have to check idle when removing that hose. never got any codes only codes I've ever had was for thermostat and thats been done.
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
ok Roadie I disconnected the hose from intake resonator did not change the idle at all the only hose that changed idle was the one on the TB and it only changed 100 rpm then when I hooked it back up truck almost stalled then idled normal.
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
Enderbygrandpa said:
dont feel any vaccuum and no soundof it also hooked up a hose and blew through it no restriction

I take it from your question that there should be vaccuum at this port when at idle. is there a connection under the resonator that needs to be hooked up that i might have missed when I have had it off?
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
the roadie said:
The intake resonator can be a dark and scary place. Not as icky as the far side of the throttle body, however. :eek:

ok can't get to that for couple days but how does this vaccuum work with the fuel pressure regulator? high vaccuum at idle lowers fuel pressure and at higher revs give less vaccuum and then raises the pressure? is this how it works? if so maybe thats why my tailpipes are always black and have been since truck was new
 

Mypetropig

Member
Dec 29, 2011
226
Just stumbled on this thread. I do not have any idle issues
but I mine will not jump to 900 either if I remove that hose. If the resonator and hose have no obstruction, then what?
Bad fuel pressure regulator? Hard to believe but, what if there is no obstruction and no vacuum?
 

BRomanJr

Member
Dec 9, 2011
371
the roadie said:
You were idling at 600, pulled this hose, and it didn't jump to 900? Do you have vacuum at the port?

View attachment 11760

Because this port is on the resonator, the amount of vacuum is based on the Throttle Body Plate position and the cleanliness of the air filter. This port will never achieve manifold vacuum.

I agree most FPRs have manifold vacuum applied to them and the discussed logic would work, but it has puzzled me why GM would plumb the FPR on the I6 to the Resonator?

Memory tells me there is a port nearby on the Intake Manifold, Are all of these Trucks plumbed wrong?
 

The_Roadie

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Member
Nov 19, 2011
9,957
Portland, OR
BRomanJr said:
Because this port is on the resonator, the amount of vacuum is based on the Throttle Body Plate position and the cleanliness of the air filter. This port will never achieve manifold vacuum.
I keep forgetting that. You're right. When people put the hose on a true intake manifold port with good vacuum, they seem to all end up with problems.
 

CaptainXL

Member
Dec 4, 2011
2,445
the roadie said:
There should be vacuum there

Why? There should only be a vacuum there at off idle and more throttle. Gradually increasing until fpr kicks in and overcomes spring diaphragm pressure. There is only atmospheric pressure before throttle body. Vacuum after throttle.
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
the roadie said:
I keep forgetting that. You're right. When people put the hose on a true intake manifold port with good vacuum, they seem to all end up with problems.

OK Roadie when you pull your hose off can u hear the sucky noise and maybe feel the vaccuum? Maybe the fpr is really sensitive to the low amount of vaccuum that is available and I was listening for normal amount of vaccuum. my ears not what they used to be. also maybe thats why my pipes are black.
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
CaptainXL said:
Why? There should only be a vacuum there at off idle and more throttle. Gradually increasing until fpr kicks in and overcomes spring diaphragm pressure. There is only atmospheric pressure before throttle body. Vacuum after throttle.

Ok then if no vaccuum there at idle, should not change anything when you remove hose.
 

CaptainXL

Member
Dec 4, 2011
2,445
Enderbygrandpa said:
Ok then if no vaccuum there at idle, should not change anything when you remove hose.

Correct. Afaik a bad fpr is either leaking or stuck and wont allow full throttle performance to increase. I suppose it could be stuck in the other position as well which would make the truck run rich at idle..

Btw my apologies to Bromanjr. Didnt see his reply which basically said the same observation how the fpr gets its vacuum.
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
CaptainXL said:
Correct. Afaik a bad fpr is either leaking or stuck and wont allow full throttle performance to increase. I suppose it could be stuck in the other position as well which would make the truck run rich at idle..

Btw my apologies to Bromanjr. Didnt see his reply which basically said the same observation how the fpr gets its vacuum.

so does your engine revs increase when you remove the hose and if so can you hear or feel any vaccuum. can u measure any vaccuum with a gauge? I'm going to see what mine shows tommorrow then get me a new FPR
 

CaptainXL

Member
Dec 4, 2011
2,445
Enderbygrandpa said:
so does your engine revs increase when you remove the hose and if so can you hear or feel any vaccuum. can u measure any vaccuum with a gauge? I'm going to see what mine shows tommorrow then get me a new FPR

I dont have a vacuum actuated fpr because that changed in 05. Your best bet is to have someone here remove the hose and block it off going to the resonator. Then apply 20 in hg vacuum directly to the fpr and see if the rpms increase. That would verify what you are experiencing.



Have you taken off the throttle body and cleaned both sides really good?

Has the fuel filter been changed as well?
 

AV8ER

Member
Apr 19, 2012
260
I noticed what ever it is that this hose attaches to on the other end is leaking fuel a bit on mine. There is another tube attached to the intake manifold right below this hose that attaches to something behind the big ac canister. This one does have a vacuum but I'm not sure what it goes to.
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
OK went out and hooked up a vaccuum gauge to the port on the intake resonator and stated the engine. nothing the gauge did not move revved the engine same thing checked the gauge it works. the part number on the resonator is 15176239 and 15047009 I don't see how this would work as the port goes into the resonator which is just hollow. hooked up a vaccuum pump to the line went up to 20hg didn't change anything, blew in the line didnt change anything.truck is running fine right now. Roadie is the part number the same on your truck? how about your truck Mypetropig?
 

Mypetropig

Member
Dec 29, 2011
226
We have had some crappy weather here lately so I will
check the p/n when I get a chance. The resonator is hollow
all the way back to the TB. So if vacuum where present it should
get it at that port. I do not have a gauge to very you findings on my truck though.
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
Mypetropig said:
We have had some crappy weather here lately so I will
check the p/n when I get a chance. The resonator is hollow
all the way back to the TB. So if vacuum where present it should
get it at that port. I do not have a gauge to very you findings on my truck though.

ok thanks just curious but is your tailpipe black as if it is running rich? my pipe is always black yet I see others that the pipe is gray or almost white on the inside. If i could clean this up maybe I could get better mileage. run between 16 to 18 highway at about 65mph for at least the last 5 years . IIRC when truck was new got about 21. truck has 125000KM's now. of course when pulling our 26 foot TT I get 10.
 

Enderbygrandpa

Original poster
Member
Jan 21, 2012
58
Enderbygrandpa said:
ok thanks just curious but is your tailpipe black as if it is running rich? my pipe is always black yet I see others that the pipe is gray or almost white on the inside. If i could clean this up maybe I could get better mileage. run between 16 to 18 highway at about 65mph for at least the last 5 years . IIRC when truck was new got about 21. truck has 125000KM's now. of course when pulling our 26 foot TT I get 10.

OK installed new FPR and engine does not change the rpm's when I unhook the vacuum line. Truck idles at 650 on the tach, runs smooth. Roadie I still cannot understand how when you unhook your vaccuum line it increases the RPM's. On my truck there is no measurable vaccuum there at idle:confused:
 

CaptainXL

Member
Dec 4, 2011
2,445
Enderbygrandpa said:
OK installed new FPR and engine does not change the rpm's when I unhook the vacuum line. Truck idles at 650 on the tach, runs smooth. Roadie I still cannot understand how when you unhook your vaccuum line it increases the RPM's. On my truck there is no measurable vaccuum there at idle:confused:

That's correct. And there shouldn't be any vacuum until the throttle is near wot. If the rpm is changing with the vacuum hose off then fuel is getting sucked into the resonator from a bad fpr. You can remove the entire air resonator including the fpr hose and it wont make a lick of difference in rpm or vacuum reading because all of that plumbing is before the butterfly valve in the throttle body. A vacuum is only pulled after the throttle body.
 

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