LT1 not running quite right
09-13-2011, 09:26 AM
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LT1 not running quite right Hey guys, new on here. Hopefully one of you can help me. I have a LT1 from a 95 Camaro that is actually in a 1984 Jeep CJ-8. This motor was already in it when I bought the Jeep, and its not running just right. Here's what its doing.....
When I go to start it, it fires right up and idles like its supposed to for about 2 seconds. Then it idles down to almost dying for about 2 seconds, then comes back up to normal idle and runs good. So then it sit there idling nice and pretty all day. But if I goose the skinny peddle a few times and then let off, it slows down to its almost dying idle speed again for about 2 seconds, then comes back to normal idle.
When I put it in drive it runs great....has lots of power! But again, if I nail the gas pretty hard for a few seconds, it wants to die when I let off.
I'm thinking a vacuum leak somewhere, but I can't seem to find one. The tach doesn't work so I can give you exactly rpms during this. Fixing that is also on my to do list that I just haven't gotten to yet.
Ive tried to hook up a scan tool to it to read for codes, but the guy that did the wiring must have been blind because it looks like a rats nest, and for some reason, I can get the scanner to connect to the PCM. Do you know of any sensor that would cause this if that sensor is bad?
Any advise, tips, suggestions would be greatly appreaciated.
Thanks,
Mike
Katy Texas
When I go to start it, it fires right up and idles like its supposed to for about 2 seconds. Then it idles down to almost dying for about 2 seconds, then comes back up to normal idle and runs good. So then it sit there idling nice and pretty all day. But if I goose the skinny peddle a few times and then let off, it slows down to its almost dying idle speed again for about 2 seconds, then comes back to normal idle.
When I put it in drive it runs great....has lots of power! But again, if I nail the gas pretty hard for a few seconds, it wants to die when I let off.
I'm thinking a vacuum leak somewhere, but I can't seem to find one. The tach doesn't work so I can give you exactly rpms during this. Fixing that is also on my to do list that I just haven't gotten to yet.
Ive tried to hook up a scan tool to it to read for codes, but the guy that did the wiring must have been blind because it looks like a rats nest, and for some reason, I can get the scanner to connect to the PCM. Do you know of any sensor that would cause this if that sensor is bad?
Any advise, tips, suggestions would be greatly appreaciated.
Thanks,
Mike
Katy Texas
09-13-2011, 09:59 AM
#2
i would check your IAC valve. that could be giving you issues if it was bad. Also, with the engine running you could try to put acetylene around the intake and see if its leaking. If there is a leak it will suck it in and idle up. Id also check the throttle body and see if its sticking closed. you might have to adjust the idle screw if someone previously did it.
09-13-2011, 12:16 PM
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Thanks for the replies guys. I'll check out my IAC.
I also just bought a vacuum line kit off ebay for $50....figured for that price, wouldn't hurt to change them all anyways.
I also just bought a vacuum line kit off ebay for $50....figured for that price, wouldn't hurt to change them all anyways.
09-15-2011, 11:52 AM
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As far as I know, the engine is stock and has a stock tune.
I just got finshed going through my engine wiring harness, and working on the body harness now. Once I get all that finished, and change the vacuum lines, I'll see how she runs. If no change, then I'll swap out the IAC and TPS.
Thanks again for all your help guys.
Mike
I just got finshed going through my engine wiring harness, and working on the body harness now. Once I get all that finished, and change the vacuum lines, I'll see how she runs. If no change, then I'll swap out the IAC and TPS.
Thanks again for all your help guys.
Mike
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09-15-2011, 02:24 PM
#9
Fuel pump? Fuel injectors? Plugs/wires? Doesn't sound like air is your problem. I would suspect fuel first and check that avenue out. What is your fuel pressure? Stock injectors? Catalytic converter clogged? Then check spark... read your plugs white/lean black/rich tan_coffee/perfect. Pull one and not all of them obviously because if its anything like an lt1 in the fbody they are a pita to pull usually #1 or #3 is easiest. I would check for burned wires... does it miss under a load? does it miss at idle? If all of these check out a ok then I would check for DTC's and make sure your OPTI is ok and then last i would suspect more of the bizarre stuff such as a PCM issue or ICM issue and even maybe a loose ground.
09-15-2011, 02:59 PM
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I have the fuel pressure set to about 42psi. As far as I know they are the stock injectors. No idea about the cat, but I was thinking about completely cutting it off all together (not worried about passing emissions, its an offroad Jeep only). Are there any concerns about not running a cat???
I haven't checked the plugs lately, but they are brand new with only about 6 hours of run time on them. But unlike the F-body's, all 8 of my spark plugs are very easy to get to, so I'll pull a few and check. It doesn't miss at all, either under load or at idle, runs great. It only idles really low for just a few seconds (to almost dying) right after it initally starts, and after getting on the gas pretty hard and letting off quickly.
I haven't checked the plugs lately, but they are brand new with only about 6 hours of run time on them. But unlike the F-body's, all 8 of my spark plugs are very easy to get to, so I'll pull a few and check. It doesn't miss at all, either under load or at idle, runs great. It only idles really low for just a few seconds (to almost dying) right after it initally starts, and after getting on the gas pretty hard and letting off quickly.
09-16-2011, 08:43 AM
#13
Fuel pressure sounds good. I would consider changing the fuel filter just as a preventative, then you can rule it out. I say this because while you might be running 42# at idle, hitting the gas hard requires more fuel. When the demand for fuel increases, sediment/debris might be getting sucked into the filter causing a drop in fuel pressure. It is a far shot, I know, but cheap enough to rule out.
Since it is an off-road only vehicle... throw the catalytic converter away. I recently read an article on this forum that indicates there is no need for backpressure on these motors for them to perform well. Someone please chime in if I am wrong, but if I were you, I would toss it. The way I see it, you will get better flow.
I would also make sure your motor was not tuned. If the DFCO was set wrong, then this can definitely lead to the problem you are having. DFCO stands for Deceleration Fuel Cut-Off.
I got this from LT1PCMTuning.com
Since it is an off-road only vehicle... throw the catalytic converter away. I recently read an article on this forum that indicates there is no need for backpressure on these motors for them to perform well. Someone please chime in if I am wrong, but if I were you, I would toss it. The way I see it, you will get better flow.
I would also make sure your motor was not tuned. If the DFCO was set wrong, then this can definitely lead to the problem you are having. DFCO stands for Deceleration Fuel Cut-Off.
DFCO
This leads us to DFCO, deceleration fuel cut off. This controls fuel cutoff when the throttle is closed and the car is moving, to enable engine braking. So, as mentioned above, when your RPMs are up and you let off the gas pedal, this table will enter play.
The way to tune these values is to log your car and determine a few things:
What is the highest MAP value reached when coasting begins?
This can vary depending on your RPM and speed, so look for the highest value whenever coasting starts. Use that value, or 1 less, for your "DFCO MAP Enable" value.
What is the highest MAP value reached when you'd like engine braking to stop?
It will usually be at least 3-5kPa higher than the above value. You will use this value for your "DFCO MAP Disable" value; but, be careful you don't set it too high, or you will run into engine stalling when coming to a stop.
In what RPM range would you like engine braking to occur?
Set the "Enable" value to the RPM above which the engine must be for DFCO start, and the "Disable" value to the RPM when you want DFCO to stop.
Lastly, how "hard" do you want the engine braking to be?
By increasing "DFCO Spark Retard," you will be increasing the rate of deceleration; lowering the retard value will make engine braking slower/smoother. There may be times when your total timing will be 0°, if you set this value high enough and your "Closed TPS Timing" values low enough. That will make for pretty fast engine deceleration.
This leads us to DFCO, deceleration fuel cut off. This controls fuel cutoff when the throttle is closed and the car is moving, to enable engine braking. So, as mentioned above, when your RPMs are up and you let off the gas pedal, this table will enter play.
The way to tune these values is to log your car and determine a few things:
What is the highest MAP value reached when coasting begins?
This can vary depending on your RPM and speed, so look for the highest value whenever coasting starts. Use that value, or 1 less, for your "DFCO MAP Enable" value.
What is the highest MAP value reached when you'd like engine braking to stop?
It will usually be at least 3-5kPa higher than the above value. You will use this value for your "DFCO MAP Disable" value; but, be careful you don't set it too high, or you will run into engine stalling when coming to a stop.
In what RPM range would you like engine braking to occur?
Set the "Enable" value to the RPM above which the engine must be for DFCO start, and the "Disable" value to the RPM when you want DFCO to stop.
Lastly, how "hard" do you want the engine braking to be?
By increasing "DFCO Spark Retard," you will be increasing the rate of deceleration; lowering the retard value will make engine braking slower/smoother. There may be times when your total timing will be 0°, if you set this value high enough and your "Closed TPS Timing" values low enough. That will make for pretty fast engine deceleration.
