I have been battling this for almost 2 months now. The idle is bad and crusing the car cuts out or stumbles. This used to be intermittent but now all the time. This is most evident under 2k rpm. Full throttle, medium and up acceleration is fine. The car feel like it has plenty of power. I did the hotcam, headers, opti and it ran great for about a month. Then I needed Ed Wright to correct my program (fans adjustment) and swapped my PCM with one of his. That is when it seemed that was the last time everything was fine. Once I got the replacement PCM, the car started to idle bad and the power was down. After 3 weeks of t-shooting, I found out the PCM they sent me did not have the knock module in it. I got one from them and the idle and drivablity was much improved. However it still was not right. This is not just a little problem. It is very noticeable.

I have replaced plugs (twice), plug wires, optispark, coil, ICM, swapped out two sets of new O2s, cleaned the throttle body and IAC, plugged all vacuum lines to test out the idle, disconnected the MAF to see if idle improved, inspected and re-adjusted rockers/valve springs, did the resistance check on the injectors, another PCM (tested today). I check a few of the intake bolts and they are tight. The way it cuts out like all cylinders make me think it is electrical. It cuts out so hard sometimes while cruising that it bangs the drive train.

I am just shooting in the dark now and am hoping that someone has experienced a problem like this. I have searched the forum on several occasions and tried a number of things as you can tell.

 

yucko...

you have been pulling harnesses apart... there may be a bad contact in one of the connectors.

 

I took good look at the PCM connectors and they are good.

Anyone know if a bad Throttle Position Sensor can cause this? I Autotaped and found the IAC shows high 20s which I hear is correct. Can the IAC still be bad? Can it affect the rpm ranges above idle if it is bad?

 

Mine did the same thing. It would happen sometimes during cruise, other times during hard acceleration. The most unnerving was during a road trip when everything seemed fine, then you could feel the slight stumble. At one point the car stumbled and died, leaving me stranded with a no-start situation until after 30 minutes when the car cranked up without a hitch!

I had on and off problems at random for about 8 months until it finally got so bad that I caould just about guess when it was going to happen. I replaced the coil, wires, plugs, tested the MSD unit, all to no avail.

I finally bought a new ECM (even though I just KNEW that couldn't be the problem) and the problem immediately cleared up - perfectly. It's worth the $90 to get a new one and try it out. If that's not the problem, at least you'll have one for when it DOES go out. From all I've read, it's a very common occurance. Good luck,

Rome

 

I have swapped out 2 different PCMs so far with no change. My next shot in the dark will be bad tank of gas. I realized last night that I always fill up at the same station.

 

Erricer, hey I've got a similar problem, stumble or miss at light throttle and irratic idle. I've been throwing some parts at it, including the IAC and TPS which didn't help. Similar to yours, it started after Hot Cam install. I'll shoot you an email, so we can compare notes.

 

OK, I will call you around 8pm. This is strange. I have a friend that got his hotcam installed and after a week or so he experiencing the same thing except he says the idle is good. His is a LT4 as well.

 

A stupid Question, but have you monitered the FP when the problems happen? I'm wondering if the pump isn't craping out on you. Maybe due to a ground / connection at the pump? I also read something in Helms about the TPS sending a false signal to the ECM. I would have to find that reference.

 

I don't know what to expect with the FP while driving. Not sure what is considered normal with the hotcam. I have not replaced the TPS sensor but the values don't just and at least what Autotap can show me.

 

I too am very interested in this subject. My car was doing very similar things. After the head cam swap and an ed wright programmed computer.

Idle searched sometimes I figured this was due to the large cam I am running. Then a stumble type miss when cruising around 2500-3000 rpm. Thought this maybe was the cam down low not making much power. Up high it ripped it hard, but not as hard as I felt it should. Then one day just crusing it backfires and dies... I am getting ready to replace the opti, I too have done new o2's new wires, plugs twice, new ICM.

Keep me posted on your findings!!!

 

I tried to find that TPS reference but couldn't. Guess I'm getting old...but as I recall it had something to do with a worn throttle plate (arm that attaches the butterflies) and the fact that it would move at idle causing the TPS to send a false signal to the ECM ....therefore an eratic idle. I'm shooting from the hip here as I need to locate the reference in book #2 Helms. I would tape a FP guage to the windshield to what the FP is when it acts up. Just a thought.

 

I'd have Ed reflash the program. Stumble and surge are usually indicative of timing issues, especially at low rpm. If I recall (notes are at home) , you add timing for stumble, for surge you remove.

Did your new PCM have the stock program in it or Ed's? I'd try the stock program again as a test (although you may get a Misfire DTC).

Jim Mason
'96 Grand Sport #007
LT4 HOT Cam, HD Meziere, Flowmasters, LT1 Edit, etc, etc

 

I am running my PCMforless PCM now and it behaves no different. Ever since I changed the fuel filter and properly tightened it down (it was hand tight) I have been getting DTCs 152, 132, 300. The first two can be related to fuel. The 300 is a misfire. I am taking it in to the local Vette mechanic tomorrow. I have had enough.

 

P0152 and P0132 are o2 sensor high voltage codes on the sensors before the cats (Bank 2 Sensor 1 and Bank 1 Sensor 1).

Could be a ground problem. Perhaps a common ground got scorched on the Long Tubes? I believe the PCM goes into "Limp Home Mode" with these codes. This would result in a rich condition, that in turn can cause a stumble.

Keep us posted,

Jim Mason
'96 Grand Sport #007
LT4 HOT Cam, HD Meziere, Flowmasters, LT1 Edit, etc, etc

 

Do you have ANY exhaust leaks around those long tube headers? If you do raw unburned air can be drawn into those leaks & trick the o2 sensors. In essence the o2 sensors will see a lean condition, and richen the mixture in closed loop. Over time it usually gets worse as the PCM keeps making long term corrections (richer) to try and zero in on the correct mixture required for the car. The real bad exhaust leaks will throw a "lean" code on the o2 sensor nearest the leak. Also make sure there are NO vacum leaks behind the MAF. Do you know anyone that has the Datamaster program/software to run on your car? :)

 

I have a stock motor (LT1) in my 94 that has just developed the same problem 3 weeks ago. It started as a miss once in awhile. The miss would cause a good thud. now it runs like crap at idle and stumbles until you pull some higher rpms, like 2000 plus. I can feel the stumble when cruising on the highway. I'm going to change the fuel pump first (changed the filter already) and see if this solves the problem. Any one try this yet?

Bill Ankerholtz

 

I'd concentrate on the 132 and 152, it's not an amazing coincident that both sides have the same code. Under Diagnostic aids on page 6-373 and 6-405 of the FSM they list problems that can set these codes. Some of the problems could have been been introduced when the cam was replaced:

- Fuel pressure too high, if fuel pressure too high these DTCs can be set.

- Rich injectors.

- Leaking injector.

- Evap emissions is saturated with fuel.

- MAF sensor, disconnect sensor and see if rich condition is fixed. Is MAF sensor in backwards?

- Breaks in the O2 sensors harness.

- Leaking fuel pressure reg diaphram.

- TP sensor intermittant will cause the system to go rich.

Hope this helps.

Eric

 

The headers are not leaking and the O2s are in the headers themselves before the flange. I have run the Autotap against it before and the O2s look like they are operating fine. The DTCs 132 and 152 happen occasionaly as with the SES light. The SES light is not on all the time. It comes and goes.

 

I have the manaul and read the conditions that can cause those codes. I was thinking that there is something going on with the injectors and want the mechanic to t-shoot that as I have no experience in that area.

I got the results from the machanic just a few minutes ago. He says there is something wrong with the PCM. The misfire histories on all cylinders are in the hundreds with only 50+ miles on the clock. The misfires occur at any steady speed with acceleration being fine. The DTCs I am getting is the PCM's attempt to address the misfires I am getting. I will put the Ed Wright PCM back in and see if it still occurs. Since the last time I did that I have changed a few things.


[Modified by ericcer, 12:55 PM 11/26/2002]

 

Hope the PCM swap corrects your problem. My o2's are also prior to/above the collector junction. I had a couple small exhaust leaks at the head/header flange. You guys may have already seen this site, but it really helps explain/diagnosis "closed loop" PCM operation. http://members.aol.com/InjuneerZZ/ScanMast.htm
Enjoy! :)