Cold, mid-RPM misfiring
05-09-2008, 11:24 PM
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Car: '91 Chevy 1500
Engine: 350
Transmission: 700R4
Axle/Gears: 3.73
Cold, mid-RPM misfiring
91 Chevy C1500 with a TBI'd vortec 5.7L
I'm somewhere near the end of my rope here. I've been getting a mid-RPM misfire while cold for some time now and I just can't seem to find out why. The misfires usually happen above 2500 RPM and goes away when its reached its operating temp (170F). If I put my foot down any more than "babying it", it misfires. Seems to completely go away when warm. I have been tuning the chip for a while, so at first I thought it could have been something I did. Well, I pulled the computer from a friends truck (that is exactly like mine, computer numbers and chip bin) and there was absolutely no change. The engine is a brand new GM crate engine (4K miles now) along with just about every other part on it, and nothing has changed the misfire. Here's what I know:
All of the plugs look identical (good) and have been changed 3 times so far
Plug wires have been changed twice (Taylor 8mm)
I purchased a brand new complete GM distributor...twice
Coil is new
Replaced the TBI
Fuel pressure is great (new pump and regulator)
No vacuum leaks
Compression looks even on all cylinders
I can't pinpoint the affected cylinder(s). It may not be just one and using an infrared thermometer on each header tube isn't pointing to anything obvious.
I am freaking lost at this point. Right now, I only know what isn't the problem.
Any ideas?
I'm somewhere near the end of my rope here. I've been getting a mid-RPM misfire while cold for some time now and I just can't seem to find out why. The misfires usually happen above 2500 RPM and goes away when its reached its operating temp (170F). If I put my foot down any more than "babying it", it misfires. Seems to completely go away when warm. I have been tuning the chip for a while, so at first I thought it could have been something I did. Well, I pulled the computer from a friends truck (that is exactly like mine, computer numbers and chip bin) and there was absolutely no change. The engine is a brand new GM crate engine (4K miles now) along with just about every other part on it, and nothing has changed the misfire. Here's what I know:
All of the plugs look identical (good) and have been changed 3 times so far
Plug wires have been changed twice (Taylor 8mm)
I purchased a brand new complete GM distributor...twice
Coil is new
Replaced the TBI
Fuel pressure is great (new pump and regulator)
No vacuum leaks
Compression looks even on all cylinders
I can't pinpoint the affected cylinder(s). It may not be just one and using an infrared thermometer on each header tube isn't pointing to anything obvious.
I am freaking lost at this point. Right now, I only know what isn't the problem.
Any ideas?
05-18-2008, 08:59 PM
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Car: 91 RS
Engine: 305
Transmission: T5
Axle/Gears: 3.73
Re: Cold, mid-RPM misfiring
Without more data it sounds like lean misfire. In other words it needs more AE and or PE while it is still warming up to temp. Try increasing the values in the areas that are giving you problems.
What do the datalogs from the WUP show? Your sig says that you have the EBL?
What do the datalogs from the WUP show? Your sig says that you have the EBL?
Last edited by DM91RS; 05-18-2008 at 09:04 PM.
05-18-2008, 09:21 PM
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Car: '91 Chevy 1500
Engine: 350
Transmission: 700R4
Axle/Gears: 3.73
Re: Cold, mid-RPM misfiring
I must have deleted the logs I had because I can't find them.
I'm not going into PE, but if AE was the problem, wouldn't putting my foot down further make it change some?
Id say if anything, there would be too much fuel. I haven't changed anything in my AE tables, but have adjusted the BPC, VE tables for my larger injectors and VAFPR.
Aside from that (before the VAFPR) I could get it to misfire with no load just by reving it up a little. It would behave the exact same way with a completely different ECM and stock chip, which is why I am thinking that the problem is not with my tuning (or lack there of).
I'm not going into PE, but if AE was the problem, wouldn't putting my foot down further make it change some?
Id say if anything, there would be too much fuel. I haven't changed anything in my AE tables, but have adjusted the BPC, VE tables for my larger injectors and VAFPR.
Aside from that (before the VAFPR) I could get it to misfire with no load just by reving it up a little. It would behave the exact same way with a completely different ECM and stock chip, which is why I am thinking that the problem is not with my tuning (or lack there of).
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