Hi guys-

I usually hang out at the c5 section as I have a 1999 arctic white vert, but I also have a 95 Impala SS with an LT1. I am having some problems with the LT1 in that the car seems to miss at idle and is shuddering and missing when you move the car. It starts and restarts with no issues; just runs rough. These problems seem different than those noted in most of the other opti threads that I have read. I have recently changed both the plugs and plug wires (ugh!!)and the car ran fine after that for a few months until this problem developed. I also changed the fuel filter. I looked at the spark plug wiring without the underhood lights and the engine running after dark and saw no evidence of sparks jumping where they should not be. The car has 235K on the odo and retains the original opti, coil, and ignition module. I spoke to a Corvette mechanic and he feels its the opti, but it seems that it could also be fuel related , possibly a bad injector? Is the ohms test conclusive as to whether an injector is bad? Given that some of these parts can be pretty pricey I am reticent to just start changing things without some degree of certainty that the parts changed will rectify the problem...help!

 

As one bean counter to another you are smart not to throw parts at an LT1. One thing to check is the condition of the coil wire particularly if its never been replaced.

 

Thanks Mr P... coil wire was changed with plugs and plug wires...

 

I'd start with some fuel treatment. My 96 was running rough so I ran some fuel treatment for water, and cleared it up. Then I changed the fuel filter. That's where the ole 10% ethenol can cause some issues. Especially if the car sits for an extended period of time.

 

A multi-meter and the factory service manual(if you post a request, someone could scan the troubleshooting section and email it to you) is the cheapest way to get this solved.

 

If you have checked fuel pressure and its in spec both with KOEO and running....and you feel the fuel system overall is ok, then I would look at the coil itself, ICM then the optispark. 235k miles is a long life for any engine part, especially an optispark unit on an LT1. The optispark is can have many symtoms of failure...not necessarily a no start condition. If you do buy a new unit, dont cheap out on a POS unit...buy a quality one. A new MSD version is a good unit, pricey, but good quality...older MSD units had some issue with the iptical sensor failing prematurely. I was able to get my MSD unit rebuilt by MSD for 150 dollars shipping included.

 

Exact symptom when my ICM dumped on my way to work.

 

Yeah, I'd say ICM as well. Damn, 235k on original opti

 

Any codes?

 

Ricasso-

It is an OBD1 car so none of the auto parts places seem to have a scanner ; I read somewhere where you could use a paperclip and count blinks or something but I couldn't get it to respond .

Thanks to all for the comments thusfar particularly the one about the ICM... is there a way to test it to see if it may be malfunctioning?

Thanks BeanCntr

 

If I recall correctly, the ICM will test fine as long as it's not hot. I belive you can get them tested at NAPA.

 

try it again. Get the paper clip pushed in good so it makes good contact with the A & B connectors. Those are the top 2 on the far right of the ALDL. Then turn the key on, the SES light should first flash a code 12 SES flashes 1 then 2 times...this will happen 3 times in a row for EACH code...then when its done finding the codes, the code 12 will flash again indicating youve seen all the codes stored...then it just keeps repeating the process. If you are getting nothing at all, as far as flashes, you have a PCM problem

 

81 c3-

Thanks for the info, I will try to read the codes again and keep my fingers crossed that it will work this time...so are all of the codes two digit numbers?

 

yes

 

You mention the car is a '95 so you need to confirm the number of pins in the ALDL. Some '95 cars have a 16 pin ALDL and the A - B jump mentioned by 81C3 wouldn't work. '95 B body I don't recall if it's 12 or 16 so check first. Jumping the wrong pins can be "catastrophic"!

If it's a 16 pin connector the later OBD2 scanner may read codes if there are any.

 

Good point. I assumed it was an OBD1 12 pin design based on my own car....

 

Finally had a opportunity to get back into this and tried to get the codes using the paper clip method with no success. The car was built in 11/94 so it is an "early" 1995 model which I believe should eliminate the possibility that it is an OBD 1.5 or 2.0. It has a 12 port connector with only three in use, the two on the upper right (a& b) and one on the far right on the second row. I tried to get the codes two ways once with the ignition off I inserted the paperclip between the A&B slots, turned the ignition to on and the cooling fan came on but the CEL was not lit or flashing. Another time I had the ignition to the on position and then inserted the paperclip between the A&B slots and the cooling fan started again and the CEL was lit but again no flashing. I had to replace the battery in the car as two cells were bad so I imagine that may have cleared the memory, but I did run the car and after a time the CEL was lit, this was last night. Before I tried the paperclip test I restarted it today and the CEL was not lit, so that seems to be an intermittent condition. Can anybody shed some light on why I can't get any codes?

In the meantime, I am going to take the ICM out and have it tested to see if that might be the culprit and if it tests bad, replace it and the coil. If that doesn't work I guess I am on to the waterpump/optispark etc.

Please advise as to your thoughts, thanks!

 

your Opti is 135% [beyond] it's designed life.. might be worth taking a look inside.

 

I agree. If nothing else its sure due a cap and rotor.

 

Maybe the EGR isn't seating Check to make sure its not open at Idle by opening it with a screw driver and see if it seats closed and if the Idle clears up