I have a ’92 Seville 4.9/4T60E in my ’88. Stickpony reprogrammed the chip and really woke it up with no issues. Last week it would not start and I discovered the white coil wire was burnt through. I found that the previous owner of the 4.9 forgot to reinstall the coil ground strap. Fixed that, new ICM, new coil, already had new plugs, wires, cap, and rotor. Now I am having trouble with a stumble/loss of power under moderate to hard acceleration. Switched ICM and then coil with good used parts and still the exact same condition. I measure the fuel pressure driving (47 psi and 52psi under acceleration – corvette pump). Fuel pump voltage 12.2 volts. Throttle position sensor showed a smooth sweep. Tried a different MAP - no change. Tech1a scanner shows no codes and coolant and air temperature were correct. Unplugged the EGR solenoid and plugged the vacuum line – no change. If driven easy – no issues. I can accelerate and bring it right to the edge of the stumble with an easy foot. It never had the smoothest idle so now what? Injectors, EGR, Igniton?
The 4.9 MAP <> the MAP used on almost every other GM vehicle. You are using the correct MAP right?
MAP vacuum line is not plugged?
How does it run with the MAP sensor unplugged?
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Does this trouble seem to happen at a specific location on the throttle? Everything fine up to that point and then almost total failure? Does this point get deeper in the the throttle as the engine runs faster? ie the problem occurs at 1/4 throttle at low RPM but at 3/4 throttle at higher RPM? If so look for a problem on the high voltage side of your ignition system. Ignition coil allowing the spark to jump to ground? You can only test this with a new coil, it won't show up on a meter. Rotor allowing the spark to jump to the shaft via a crack in the rotor? Look for a black spot on the rotor.
This problem can be caused from high resistance in old spark plug wires, plugs gapped too wide, or running the engine with spark plug wires disconnected. If the high voltage is jumping through the coil to the low voltage wires it can kill your ICM.
If the tach wacks out when this problem happens for sure look for high voltage secondary to low voltage primary arcing.
[This message has been edited by phonedawgz (edited 08-23-2014).]
The stumble seems to be load related - the harder you push, the more stumbling you get and that's throughout the rpm range. The map sensors tried were the stock 4.9. The ignition parts are new and the only part that had carbon was the coil button contact on the rotor (before coil replacement). The coils used were both ohmed and in spec. The tach does twitch violently downward when stumble occurs..
Had the same problem with a mustang once. The connector inside the sparkplug boot on one of the distributor posts was out of position and sitting across the top of the hole and not in the post's hole making proper contact with the part that receives the spark from the rotor. The car drove normally but when pushed a bit, the engine would missfire and stumble
Which is why you need to run the wire back there to the dist to measure the voltage while driving.
If the voltage to the dist stays up there then I am leaning towards the secondary again and perhaps the same problem is happening with two coils. But check the voltage first.
What white wire went bad in your coil? The white wire under the cap from the coil to the terminal? Or the white wire outside the dist that is marked TACH and runs to the engine harness?
I know you said you already got new plug wires, but it is possible that one was bad from the factory. Had the same issue with my 3800 and that's what my problem was.
I have yet to figure this out. I gave the car to my son so it is gone a lot. Drives fine when you don't floor it. I did replace the injectors with used Bosch III units ( I planned on doing this anyway - nice upgrade btw) and it runs a lot smoother but still has the same problem. I don't have the PBJ turbo engine. (I wish!) When I get a chance I will check the spark plugs (E3's) and the plug wires. Just because they are pretty new I shouldn't assume they are good. I measured the voltage to the distributor at idle and it was 13.7v. I have yet to jumper that wire to test the voltage while it is stumbling. This thing pulled strong when I completed the swap which has got me pissed. I can usually figure this stuff out but I am still stumped.
Don't you have the turbo 4.9 that was once in her86gt? If you still do you should contact PBJ here on the forum. He built it originally.
Yes, I've still got it. Fired it up tonight as a matter of fact, that thing sounds so bitchin' I can't wait to get it out on the road. My garage has been taken over by my business until a week ago so now I have room to work on it again, yeah!! I am having some difficulty with it but I was checking this link more for another Fiero I got with similar issues.
Serge, I am certainly no electrical wiz but after reading thru this again the thought occurred to me that maybe because that one wired fried, maybe something else got fried as well and that is what is now causing the issue. Even though you replaced the cap and many other parts one of them could have been fried at the same time as the wire. The one guy posted about how replacing his cap solved the issue on his Mustang so maybe it is a bad cap or plug wire, etc..
I have yet to figure this out. I gave the car to my son so it is gone a lot. Drives fine when you don't floor it. I did replace the injectors with used Bosch III units ( I planned on doing this anyway - nice upgrade btw) and it runs a lot smoother but still has the same problem. I don't have the PBJ turbo engine. (I wish!) When I get a chance I will check the spark plugs (E3's) and the plug wires. Just because they are pretty new I shouldn't assume they are good. I measured the voltage to the distributor at idle and it was 13.7v. I have yet to jumper that wire to test the voltage while it is stumbling. This thing pulled strong when I completed the swap which has got me pissed. I can usually figure this stuff out but I am still stumped.
Serge, how many miles are on the engine? if it has high mileage, it could be slack in the timing chain, which is easy enough to test on a 4.9L. ive seen this happen on another 4.9L engine before with high mileage, the problem sort of developed over time
The tach does twitch violently downward when stumble occurs..
A slack timing chain won't cause the tach to twitch violently when the stumble occurs. This is an ignition problem.
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I measured the voltage to the distributor at idle and it was 13.7v. I have yet to jumper that wire to test the voltage while it is stumbling.
13.7 at idle - is this below what the voltage read at the battery? If so it sure sounds like that is your problem. You should be reading something close to the voltage at the battery (14.7) My guess is when the load is higher (when the coil has to put out 2x or 3x as many sparks per second) the voltage drops even further..... BUT before fixing something that you aren't sure if it exists or not.....
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Originally posted by phonedawgz:
Run a while back to the distributor for testing and check the voltage on the BAT terminal of your distributor when the problem is happening.
[This message has been edited by phonedawgz (edited 09-21-2014).]
