Hello I Have A 91 Vette with a Poor Idle And a Failed Emission test.I was wondering About Some Options You Guys Might Know About. Last Summer I Changed The Intake Manifold Gaskets, While Doing That I Did A Major Tune-up Plugs,wires,cap,rotor,pcv Valve,air,fuel Filters,transmission Fluid And Filter Change And Changed The Coolant. Also did a Full Cleaning of The Throttle Body. I had a Code 32 before the work so I changed the Egr Valve.The car stalled on me a few times last year before the work does not stall now,just Rumbling Like Its Gonna Shut Off At Lights,in Traffic Etc.I Used A Gm Service Manual Throughout The Process And Ordered All The Necessary Parts Through The Last Detail Atlanta Corvette, Torqued Every Bolt To Spec. My Name Is Mark From Conn. I looked At This Site Before And Its Very Helpful Just Trying For Some Insight On The Problem. Also No Check Engine Light Or Codes Coming Up, I Told My Mechanic That I Was Gonna Take The Injectors Out And Send Them To Get Cleaned,But other vette owners said it's probably not the injectors.They said it could possibly be some sensors o2,mat,iac,egr solenoid.Thanks.

 

I'm pretty sure GM was still using the problematic Multec injectors in '91. It's quick and easy to check the injector resistances, just pop off the harness and measure the resistance across the two prongs on the injector. If any are significantly less than 15 ohms, then you could definitely have a problem there.

Also, pull the spark plugs and read them--if you've got an injector or two that's bad enough that you're failing emissions, I'd expect the plugs not to be consistently colored.

Good luck!

Ben

 

I wouldnt bother cleaning the Multecs, they are age old and may fail soon anyway. five-o-motorsports sells new ones for around $300. Make sure your IAC and TPD ius adjusted properly, no vacuum leaks. If your EGR.egr switch/solenoid is good you should pass with no problem.

If an injector is partially plugged and causes it to run even too lean you can fail smog.

 

They had L98's in '91 right? if so, what is your timing set at? Also go to autozone and buy a bottle of RXP and dump it in.

Punctuation helps us read easier too.

 

That is all, one long, very hard to follow, sentence. Except for periods at the end of abbreviations, the only one was at the end. And with 97% of the words capitalized, it isn't easy detecting when switching to a different thought(?). Since you say you have no codes, maybe you would care to share the results of the failed smog test, along with the standards.

RACE ON!!!

 

Hello I agree didnt pay much attention to the typing trying to say everything thoroughly.The test results I had were HC (PPM) Limit 53 Emission 51 Pass CO Limit 0.28 Emissions 0.53 Fail NOx Limit 614 Emissions 218 Pass.

 

The CO means it's rich and with the poor performance, you need to check all the things that can make the ECM add too much or too little fuel to make it run right. That includes the injectors, air pump circuit, vacuum and plenum leaks, fuel canister purge and primary and secondary ignition. I'd start with the easy stuff. Clamp off the air hose to the left header and plug the fuel purge hose at the TB with a vacuum cap. If everything smoothes out, remove one or the other to find out which one is the problem. If no joy, inspect all vacuum hoses and connections and/or sniff around the engine compartment with a little bit of propane using a plumpers welder (don't light it!). If engine RPM increases, there's a leak. For the injectors, it's best to check how much each drops the pressure when energized, but that takes special equipment, so you can pull the rail with the injectors attached and with a paper towel underneath it, see if any drip some fuel once you energize the fuel pump - you are going to need to wait up to an hour or two to make sure that none are leaking. Otherwise, energize the rail 5 or 6 times by turning the key on and off and later in the day, start pulling plugs to see if any have fuel on them.

You can start the Primary and Secondary ignition checks can by observing your wires at night or misting them with some water. The guts of the distributor usually means taking it apart, but you can check the pickup coil output by disconnecting it and putting your DVM (set on the AC scale) across it's terminals. Then while someone cranks it, check for at least .5 volts AC. Module should be tested at a Dealer or an autoparts store assuming they even have a tester.

 

If it was all said thoroughly, it would be difficult to tell, as hard as it was to slog through that mess.


I doubt he has anything seriously wrong with the ignition system. While the CO is high, indicating a rich mixture, the HCs are within limits (barely) indicating that most of the excess fuel is being burned.

RACE ON!!!

 

I just had my 86 inspected last week. The first time, the CO was 1.6, the limit being .97. I changed the timing from 8 degrees to 6, and put a bottle of that RXP to 10 gallons of gasoline. I burned a quarter tank out and got it re-inspected, .96 CO.

As far as the sensors you mentioned, it could very well be the O2, or maybe the coolant temp senor. Highly unlikely your problem is because of the other sensors you mentioned

 

Good luck! I'm getting some work done today and an emissions test again today. (I flunked a few times and replaced all that stuff you mentioned and some more...if I can't get through now the TPIS headders may have to go!)

 

Mark from Conn, I see that you have a FSM and that you have done a lot of work to the intake side of things including the MAF and throttle body. I do not see you stating that you did a minimum idle air adjustment. In my 88 FSM the procedure is listed in Section 6E3-C2-12 under "Minimum Idle Speed Adjustment". There is detailed IAC supporting information in section 6E3-C2-4 for reference. I would assume that your FSM would have something similar since you have an L-98 as well. I'd also double check that you are running at the correct timing value. You might try this and report back what your results or problems with the idle air process are.

If you are running a bit rich as CFI-EFI has pointed out is shown in the Emmissions Test results, and you have no codes, then you may have a component problem caused by a "slightly" defective part. I.E. a slightly leaky injector or an O2 sensor that is less accurate than it should be. A fuel pressure guage will allow you to check fuel pressure drop off due to a leaky injector or a possible problem with the fuel pressure due to a defective regulator.

You really need a scan tool to determine if the O2 sensor is operating fully up to spec. An O2 sensor (only one in the L-98) can slowly go bad as its tip gets covered with exhaust by-products over time or just due to age. (You try working in a 600*F environment for years.) While still within spec enough to not set a code, it's accuracy can become degraded enough to mislead the ECM into pulsing the injectors in a way that causes an improper mixture. Just because it is working, doesn't mean that it is working 100% correctly. I hate to recommend throwing parts at a problem, but if you cannot get a scan tool on it and the O2 sensor has above 25k miles on it, or you don't know its age, I'd replace it.

 

Thanks for the replys the car is going in the shop tomorrow to a guy that owns and knows how to fix -em.Hopefully he can diagnose the problem and get me passed and the idle problem solved.I was referred to this shop by another vette owner so we'll see what happens.I'll post the results thanks.To ctyank2 my next thing was corvette center in newington but I found someone closer let you guys know thanks again.

 

Good luck!

 

Hello Guys, I got the car back from the shop on Thursday.He checked a bunch of things and came up with a cracked Power Brake Booster and bad vacuum.The car passed emissions and does idle better maybe sometimes better to find a real mechanic than a backyard like myself. Anyone hear of that problem and remedy before.The results were HC(ppm)limit53 emiss 24 CO LIMIT0.28 EMISS0.07 NOx(ppm)limit614 emiss227.Thanks talk to you soon.

 

That's a really clean running car. Great emissions numbers but I'm not sure about the limit you listed for HC. My emissions sheet for 4/21/06:

HC 36, limit=88 (you have 53 for the limit?)
CO 27, limit=28 ( )
NOx 355, limit 614

Of course my results should be higher because I have no pre-cats and TPIS headers.

 

Excellent! Sounds like you found a great mechanic. Congratulations.