- Jul 6, 2014
- 9,265
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.
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.