OK, here are the specifics:

This is what my car is doing; about 6 weeks ago my LT1 / M6 Firebird started to run rough. I'm an engineer who was with GM back when the Tech 2 was being developed (I was a transmission calibration engineer) and, to me, the old LT1 seemed like it was running in open loop or something along those lines. Poor gas mileage, slightly rougher idle than normal (FYI, it has an LT4 Hot Cam kit) and reduced power thru the entire RPM range. It never broke up at the top end, but it just didn't feel 'strong'. Driving along at low RPMs in sixth gear was now painful too; it would shudder and shake in 6th below 2k - something was not right. No codes though... I was perplexed.

I didn't have access to a real scan tool, so I started guessing at would could be the issue; I replaced all 4 of the O2 sensors and the Engine Coolant Temp (ECT) sensor, the one in the water pump, not the one in the radiator (for the gauge) - No change.

I purchased some OBD II S/W and a ELM 327 H/W interface (over the net) but before I rec'd it, I got a job in NC and had to leave my family in Detroit. My wife doesn't drive a stick, so I couldn't leave this car for her. With great trepidation, I headed off for NC in the bird.

The vehicle drove, just not very well, like 85% or so. I hoped the 600 mile trip would finally set a hard code so I could drop by Autozone and have them tell me "You've got a <insert your favorite driveability problem here> code". I never got any DTCs. (drat!)

So, again, I started guessing; the Opti was only 20K miles old, but it was a stock GM replacement. My MSD box (6 BTM, for blowers and turbos with boost retard - the car is still N/A [I have not installed the STS yet] but the MSD still helps), plugs, wires, and MSD coil were also all about 20K miles old too. I considered trying to drive home on Easter weekend and replacing the Opti with a billet MSD unit, but (as I was driving from my new office off to lunch) my car died going up a hill. It was towed to a local Pontiac dealer in greensboro last week Thursday.

Today, the technician said he could only communicate with the vehicle via the Tech 2 for about 30 seconds before communication would cease. According to them, that happened 2 times. They wanted to through a PCM at it, but that would wipe away all improvements I've got from Bryan Herter and PCM 4 Less, and it might not help the car in its current configuration.

I don't know what to do, so I sent the PCM back to Bryan Herter, and he told me (off the top of his head) that I needed an O2 sensor - except I replaced ALL of them as a first fix. He thinks the PCM might be at fault, but the reason for my post is to ask for any additional insight from this community.

Has anyone else ever had a similar problem? What was the resolution?

Let me know if you have any suggestions - Thanks!

Mike Eldridge (great421@yahoo.com / 313 268 7931 Cell)

 

No thoughts? I could realy use a hand...

 

could be any number of things. clogged fuel filter or injectors, or clogged cat. worn out spark plugs or wires, one or more of the sensors could be bad. etc

 

Let's see, the 1992 and 1993 LT1's were speed density cars then they went to the MAF in 1994. And the gauge sender is in the driver side head not the radiator. Just thought I'd point that out.

Anyway, have you checked the coil? With no DTC, that's the first thing that pops into my head. And like they mentioned above, the fuel filter too.

 

what about the egr. have you checked that

 

dhdenney -

Thanks for your suggestions, I'm pretty sure the (new MSD) coil is ok, but it's worth checking; as for the fuel filter, its new too.

One point of mid-model / year-to-year change / engineering change request (ECR) order - I'd almost bet you money (and I don't bet) that the engine coolant temp (ECT) sender for the gauge cluster on my vehicle is indeed located in radiator, but I'll confirm this (questionable) fact when I get the car back from the dealership. I'll disconnect the weather pack connector and then check the gauge reading.

BTW - "Flow Pontiac" in Greensboro NC has been more than helpful; when my PCM needed to get sent back to PCM for Less, Flow gave me one of their cars to drive while my 11 YO Formula was at their sevice bay. I can't say enough good things about them.

Oh, and that Hottie (under your name) is sweet!

MIKE

 

Yes, Jennifer Hurt. She is so bad. I have many pics of her. Anyway, the temp sender being in the radiator is one I haven't heard, but I'm not saying it isn't true. I didn't see anything in the index of my Chilton manual. I just thought the gauge reading came from the driver side head on all of them. Those MSD coils can be tricky. I had one once that gave out in just a few thousand miles.

 

Quick Update -

The Tech says cylinders 2,4,6, and 8 are getting spark but not firing / misfiring so it must be a clogged cat - I ordered a replacement y-pipe with cats from MagnaFlow. I'll have it installed next week and keep everyoone informed.

 

Hmmm. A clogged cat is understandable for the loss of power. But it doesnt explain why the left bank alone is missing. If it was missing it would have thrown a code. Did the tech say weather or not the injectors on that bank were working?

 

Slow6 -

Good question; here's a bit more info:

The cats (all 4 of them!) are brand spankin' new, and the PCM was just reflashed / replaced by Bryan Herter @ 'PCM 4 less' last week, and installed just the other day by Flow Pontiac.

So, given that info, the car has been driven ZERO miles post the PCM installation, hence the lack of a Cat code (not enough run time / key cycles to actually trip one).

Friday, the GM Tech at Flow Pontiac did in fact state that ALL the injectors were working as well as ALL the spark plugs and Opti. His diaganosis of a clogged cat was stated as "quite plausable" by my step-father (a GM / ASE certified Master Mechanic) too.

I just hate a diagnosis by elimination, that's all...

As for why just one bank (drivers / pass) is missing, if only one cat is plugged, then that would make sense. As for why I'm replacing both cats; it just seems to make sense to replace them in a pair, especially if the aftermarket tube / cat ***'y flows X% better than stock - you want even backpressure right?

I'll keep everyone informed of the outcome.

MIKE

 

Im having the same problems (loss of power, 6th gear shaking under 2k) I just had the opti, wires, plugs, and coil replaced. My cats are hollowed out and im actually thinking that that could be my problem. Also when i get on it, it makes a really annoying noise (almost like a poping noise but its not a backfire). Im going to have my hollowed out cats removed and just put pipes in. Hopefully that would at least solve that annoying noise problem. If it solves the other problems then i will let you know.

 

Silver97TA -

Before you do that, check out my mirror thread in the PCM diagnosis forum

https://ls1tech.com/forums/pcm-diagnostics-tuning/689211-lt1-running-rough-reduced-power-poor-mileage-no-dtc-codes.html

Look at what Old Geezer said, you might have a bad FPR, dumping fuel into your intake manifold.

Keep in mind, with gutted cats, you cannot have a backpressure issue, so I wouldn't look to your exhaust for a solution. Check out your fuel system and PCM since you've already addressed all the spark components.

BTW - A bad PCM may or may not set an internal code; How can you expect a broken part to accurately / correctly diagnose itself?

Good luck!

MIKE

 

I will throw my .02 in on my old motor it would shake from in 6th gear if i was going under 70mph or under 2k like you stated. It also progressivly got worse with the way the car was running and loosing power all through the powerband.... Changed out the opti and the problem was solved for me. Don;t know if that is what is wrong with yours but it is worth a shot

 

Sounds like you're well on your way to getting this thing. As a hindsight, the question raised on the injectors made me think of the fuses for bank 1 and bank 2 injectors. Although that was not the problem, it sounds like it very easily COULD have blown an injector fuse for that bank. Based on the clogged cat assumption, it seems that the trapped exhaust gases were blowing out your spark. Good luck!

 

dhdenney -

I'd hate to think that this entire episode could have resolved by replacing a fuse! Don't scare me like that!

MIKE

 

Sometimes the seemingly impossible can be as simple as a fuse. A few weeks ago I was driving my car when it died on the highway in front of a gas station. I spent several hours tracing down stuff and in the end it was a blown 10 amp ignition fuse. BTW, I think Flow Pontiac/GM you had your car at is the dealership that operates GMpartsdirect.com.

 

OK,

The Tech replaced the stock y-pipe with a unit from MagnaFlow - No change in the way the vehicle performed! Arrggghhhh!!!!! (I'm glad I didn't spend the $1400 GM wanted for their parts!)

Anyway, the car is now getting pass side misfire codes on cylinders 2,4,6 and 8.

I'm pretty sure the Opti would not discriminate and just crap out on one set of four cylinders and leave the other four alone, so all I can think of is the pass side fuel rail being obstructed - does anyone else have any guesses?

It has been one month since my car entered the dealership, and they still can't fix it! I'd really appreciate any assistance - Thanks!

MIKE

 

This is dumb but i have to ask. Did you check to be sure the belt didnt eat up the passanger side wires were they wrap around the block?

 

I'll ask, but four bad wires seems like a long shot.

The Opti (in total) was my first thought, but they maintain that it is working properly. What could cause one bank of cylinders to misfire but leave the other bank alone?

 

Poor ground signal from the pcm for that bank. ??? I dont know man. Like you mentioned before pluged up rail. but that doesnt make sense either because the pressure would spike if that happened. Sorry i cant help more. If i think of something i will let you know.