'95 Caprice LT1 problems- HELP!
05-09-2010, 08:58 PM
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'95 Caprice LT1 problems- HELP! Sorry in advance for the long post, but I need help quick, so I'm going to try and give a detailed explaination of what's going on. About a month ago, I was driving along about 30mph, and the car all of a sudden starting running incredibly rough, and felt like it had no power. A friend of mine helped me tow it home. I'd been having problems with the car not getting any spark when it rained, so I suspected the Opti was bad. Did a fuel pressure check, and the pressure was only 25psi. This weekend I replaced the Opti (w/ all seals), plugs, wires, Opti vent harness, water pump (old one had bad bearings), fuel pump, and fuel sending unit.
After all that the car still does basically the same thing. The idle in park feels very strong and very smooth sometimes, other times it feels rough. Putting it in gear it's rough no matter what. One thing did improve- before I did all this, in gear it would ping alot at idle, and turning the steering wheel would make it die out- all of that had now gone away. It still has a LOT of trouble accelerating. It also seems like the trans shifts strange- almost like it doesn't want to shift. It seems smooth enough once it finally does though, and it was completely rebuilt just under 2 years ago.
I did swap out the coil on my car with a friend's that was a known good coil, and that didn't make a difference. I also bought a new ICM and installed that, and that also didn't make any difference. I am going to return the ICM since that doesn't seem to be the problem. It does run exactly the same in open and closed loop, so I assume that means it's not a problem with the O2 sensors? The car has had an exhuast manifold leak for awhile, which I know can throw off my knock sensors, but that problem has been around awhile and it ran ok even with the leak. One strange thing I noticed- I do have an ALDL cable, and when I run it with Free Scan, it always says the desired idle is in the 3200rpm-range. Not sure if that's a sign of a bad PCM or if the Free Scan just has some sort of glitch. I'm out of ideas, and I'm pretty frustrated. I'd appreciate any advice you guys have!
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After all that the car still does basically the same thing. The idle in park feels very strong and very smooth sometimes, other times it feels rough. Putting it in gear it's rough no matter what. One thing did improve- before I did all this, in gear it would ping alot at idle, and turning the steering wheel would make it die out- all of that had now gone away. It still has a LOT of trouble accelerating. It also seems like the trans shifts strange- almost like it doesn't want to shift. It seems smooth enough once it finally does though, and it was completely rebuilt just under 2 years ago.
I did swap out the coil on my car with a friend's that was a known good coil, and that didn't make a difference. I also bought a new ICM and installed that, and that also didn't make any difference. I am going to return the ICM since that doesn't seem to be the problem. It does run exactly the same in open and closed loop, so I assume that means it's not a problem with the O2 sensors? The car has had an exhuast manifold leak for awhile, which I know can throw off my knock sensors, but that problem has been around awhile and it ran ok even with the leak. One strange thing I noticed- I do have an ALDL cable, and when I run it with Free Scan, it always says the desired idle is in the 3200rpm-range. Not sure if that's a sign of a bad PCM or if the Free Scan just has some sort of glitch. I'm out of ideas, and I'm pretty frustrated. I'd appreciate any advice you guys have!
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05-10-2010, 10:50 AM
#2
Did you recheck the fuel pressure after the pump change? Not as familiar with the Caprice platform, but I think it has an external fuel filter, near the passenger side rear wheel. Did you replace it?
A 2 minute experiment that might be worth doing: remove the belt and see if there's a change. Usually you'd hear belt chatter/ squeak if you had an alternator, PS pump, or tensioner pulley starting to bind/ seize, but not always.
A 2 minute experiment that might be worth doing: remove the belt and see if there's a change. Usually you'd hear belt chatter/ squeak if you had an alternator, PS pump, or tensioner pulley starting to bind/ seize, but not always.
05-10-2010, 03:12 PM
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I had changed the fuel filter just before I tested the fuel pressure the first time. I actually just did the fuel pressure test again now that I have a new sender and fuel pump, my fuel pressure is 42psi, so the new pump solved the fuel problem. It idles good out of gear, and in gear is still the same.
05-10-2010, 04:16 PM
#5
Is the SES light on (forgot to ask)?
If spark is good and fuel is good and you don't have any vacuum leaks (you checked, right?), then air is likely your problem, especially given the desired idle FreeScan is reporting.
Next steps I'd suggest:
1) check the TPS with a voltmeter; should be 6.7V at the middle wire with the key on, engine OFF.
2) check the MAF for dirt on the elements, unplug it and run the engine. If no change, then MAF might be your problem. If the SES light isn't on, unplugging the MAF will likely make it come on pretty quick, just to let you know... if it's dirty, clean it with electronics cleaner. Don't use carb or brake parts cleaners on it.
If spark is good and fuel is good and you don't have any vacuum leaks (you checked, right?), then air is likely your problem, especially given the desired idle FreeScan is reporting.
Next steps I'd suggest:
1) check the TPS with a voltmeter; should be 6.7V at the middle wire with the key on, engine OFF.
2) check the MAF for dirt on the elements, unplug it and run the engine. If no change, then MAF might be your problem. If the SES light isn't on, unplugging the MAF will likely make it come on pretty quick, just to let you know... if it's dirty, clean it with electronics cleaner. Don't use carb or brake parts cleaners on it.
05-10-2010, 04:47 PM
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I can't find any vacuum leakes. I assume the spark is good, the opti is a brand new Delco unit, the plugs and wires are also brand new Delco. The injectors are only about 4 years old, all 8 were replaced also with Delco. The fuel pressure looks good now as well. I unplugged the wiring connector from the MAF and it ran the same. I will check both the TPS voltage and the gear selection on Datamaster when my new laptop charger comes in.
05-10-2010, 06:47 PM
#7
Sounds like you have a bad MAF. 
Will your friend that loaned you the coil let you try his MAF? Unplugging yours should have made it run worse (or just bad if it was running fine).
Our F body cars default to speed density mode if the MAF is unplugged. In speed density mode, the PCM just uses the low resolution signal from the Opti + IAT + MAP sensor to calculate injector pulse width when the MAF is not working or is unplugged. Most of us don't ever tune our cars to run in this mode, so they run like crap when you unplug the MAF. I don't think B body cars are factory tuned any differently to use SD mode when the MAF goes out/ is unplugged. Therefore, I am assuming that your car should have the same associated behaviors with the MAF when unplugged. That being said, if there is no change when unplugging the MAF (bad driveability with no change), then you can conclude the MAF is most likely at fault...
Will your friend that loaned you the coil let you try his MAF? Unplugging yours should have made it run worse (or just bad if it was running fine).
Our F body cars default to speed density mode if the MAF is unplugged. In speed density mode, the PCM just uses the low resolution signal from the Opti + IAT + MAP sensor to calculate injector pulse width when the MAF is not working or is unplugged. Most of us don't ever tune our cars to run in this mode, so they run like crap when you unplug the MAF. I don't think B body cars are factory tuned any differently to use SD mode when the MAF goes out/ is unplugged. Therefore, I am assuming that your car should have the same associated behaviors with the MAF when unplugged. That being said, if there is no change when unplugging the MAF (bad driveability with no change), then you can conclude the MAF is most likely at fault...
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05-10-2010, 06:58 PM
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I'm sure he wouldn't mind if I borrowed his MAF. I think I'll do that. A lot of B-Body guys swap to the F Body MAF, I think if that is indeed my problem, I'll do the same.
05-10-2010, 07:06 PM
#9
MAF diameter is different if you swap- 3" to 3.5", so you'll need the MAF housing. The sensor itself is just the black piece in the middle. I'm trying to picture the intake elbow on those in my head. Not sure you won't have to lightly mod the elbow to squeeze it inline. Also, you'd need to reprogram the MAF tables in the PCM to accomodate it correctly, IIRC. Might just want to wait on changing it to an F body MAF, since it's not creating a restriction on an otherwise unmodded car at this time.
05-10-2010, 08:11 PM
#10
unpluggin the maf just makes it run in open loop..it wont make the car run worse..theres plenty of people who drive the cars for a week or some days with the maf unplugged..i drove mine for a couple days unplugged and there wasnt a big diff..
did you check out your i.c.t or cat like everyone told you on yb?
did you check out your i.c.t or cat like everyone told you on yb?
