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1992 LT1 Coupe: First time with scan tool

You've just spent money for a new set of injectors you may not have needed. I may have suggested this before but, if I did not...you need to stop guessing and get a copy of the Factory Service Manual and troubleshoot with that.

Keep in mind that with 160 on the right side long term fuel trim, the system is adding fuel...a lot of it. With 140 on the left, it's adding fuel, though not as much.

Gven the past symptoms, that new injectors, new IAC and a clean TB did nothing and current symptoms are lack of idle stability and high fuel trims, you need to get back to basics....look for a vacuum leak.

Hib,

Perhaps my last post wasn't worded very clear. The new injectors DID improve my BLM readings, at least on one side. With the old injectors, at idle, my right bank reading was 160. With the Bosch IIIs, my right bank is now 128. It is with the left bank that I'm now seeing BLM at 140, whereas it was around 130 with the original Multecs. In addition, with the new injectors I have noticeably more power and the car runs perfectly throughout the RPM range, with the exception of at idle. All that I was stating in my previous post is that while the Bosch IIIs have produced a noticeable improvement, they didn't solve every last little issue as I thought they might. So, I have more troubleshooting to do to make it perfect.

I DO own a factory service manual and I HAVE been using it extensively to troubleshoot this. The engine was tested with a smoke machine last fall by a mechanic whom I trust and no vacuum leaks were found. He and I both have gone over all of the various vacuum lines, big and small, and checked them for proper condition and secure connections. The engine holds a steady 15" inches of vacuum at idle. If this were a traditional vacuum leak problem, I don't think that I would be experiencing an idle that is right on the money when the car is started cold, for a period climbs to 150-200 RPMs over the desired idle that the ECM is calling for after the engine gets hot, and then returns to normal RPM several stoplights later.

So far, every "guess" that I've made has resulted in improved driveability. When I first purchased the car last fall, it idled at 1,100 RPM in gear. We tried first to clean the IAC and its passageway, but that offered little improvement. After replacing the IAC, idle improved dramatically. Based on the right bank 160 BLM that I had before replacing the Multecs, my "guess" that a fouled injector was creating a lean condition proved to be right on the money. Believe me, I'm not just going out and replacing parts willy-nilly just based on a hunch, and every part that I've replaced has come after a significant amount of testing and research before plunking down cash. For instance, one might expect my symptoms to possibly be originating from the fuel pressure regulator. However, I have tested that component per the procedure in the FSM and the results show that it is working fine. Therefore, I will not be replacing the FPR. I was also suspicious of the cannister purge solenoid. After following the FSM test procedure, it showed that it was working normally as well. The original part remains.

We are talking about a nearly 19 year old car that was known to have been equipped from the factory with certain componenents of questionable quality. In addition, the car itself received very little mechanical care from its previous owner. The replacement of various parts is going to be expected, wouldn't you think?

Respectfully,

Ron
 
Vacuum leak.

Follow this test to a 'T':

Originally Posted by Schrade
No one got the proper technique for vacuum leak detection... :confused

FIRST (fully warmed up motor); block the pedal, or block the throttle linkage. YOU HAVE TO DO THIS TO GET A STEADY IDLE , around 1,250 rpms, or a little higher. If you don't, you can fish starting fluid, a propane hose, or WD40, or Krylon, or hair spray, or pee, anything, until you need a sweater in Hades, but if the motor is lopin' already from a vacuum leak, you ain't gonna' find any vacuum leak, unless it's so big that Little Jack Horner can stick his thumb in it and pull out a pineapple NOPE.

AFTER you get a steady idle, THEN, you can spray starting fluid everywhere - that's the best detector, because it's vapor pressure is very high - a LITTLE at a time, until you get the SURGE. Follow the vacuum lines to the EVAP cannister too. You might even have to follow the EVAP lines from the EVAP cannister to the gas tank too, but that vac leak will show up as a DTC 32 EGR fault, and won't normally show as loping idle, unless the EVAP cannister has been by-passed (been there, saw that uh-huh).

If you get a surge in a tight spot, and can't tell exactly where it is, light a book of matches, blow out the match heads while they're still burnin', and feed the smoke to the tight spot.

Fear not the starting fluid; you cannot put the can down fast enough and light the matches fast enough to catch the starting fluid, because the vapor pressure is so high.

Bookmark this post YUP.

Wrench Wizard OUT!
 
I have been working with almost the identical issue of varying idle. Replaced the injectors with Bosch this week. The o rings on the old ones were shot. This could account for some of the vacuum leak. Car does run MUCH better but still idles wherever it feels like at the moment. (perhaps some midol will help) I have read some posts about K&N air filters and how they can cause problems, so I'm gonna try an OEM. I am beginning to think the throttle body might be bad and have a vacuum leak somewhere. It looked great when apart but...and there are only 73,000 on the clock so it shouldn't be worn. I have looked for vacuum leaks every way possible. Blocked off lines one at a time, checked torque on the intake, replaced the pcv and all rubber attached to it, stood on my head, cant find a leak anywhere. Also checked electrical connections on the temp sending unit, MAP, and IAT sensor. Going to recheck with the scan tool this afternoon and see what it tells me.
 
I have been working with almost the identical issue of varying idle. Replaced the injectors with Bosch this week. The o rings on the old ones were shot. This could account for some of the vacuum leak. Car does run MUCH better but still idles wherever it feels like at the moment. (perhaps some midol will help) I have read some posts about K&N air filters and how they can cause problems, so I'm gonna try an OEM. I am beginning to think the throttle body might be bad and have a vacuum leak somewhere. It looked great when apart but...and there are only 73,000 on the clock so it shouldn't be worn. I have looked for vacuum leaks every way possible. Blocked off lines one at a time, checked torque on the intake, replaced the pcv and all rubber attached to it, stood on my head, cant find a leak anywhere. Also checked electrical connections on the temp sending unit, MAP, and IAT sensor. Going to recheck with the scan tool this afternoon and see what it tells me.

K & N will only cause problems if the element is over-oiled. It will foul the filament in the MAF. A couple of scans in sequence will reveal fouled MAF, IF the filter was recently serviced.

If it WAS, pull it, clean it, oil LIGHTLY, clean the MAF CAREFULLY (the filament is fragile like a light bulb filament).

Did you do the vacuum test with a STEADY idle, exactly as described?
 
This is an LT1 and as far as I can tell there is no MAF. Vacuum is rock steady and over 20 Hg. The Idle is low when I first start it. Then half a mile down the road at a stop sign it goes up to about 1400. After driving another couple of miles it drops down to about 800. Sometimes if I tap the accelerator, after it reaches operating temperature, it drops down to normal.
 
Well, with all of this banter it is clear everyone is an expert. Anyway, I will finish with my solution as my car is now running like the day I brought it home in 1992. Remember, I started last summer with a drive to work on my 92 with 20,500 miles on it. Perfect on the way to work but could not even go 60mph on the way home. Checked everything I could think and listened to everyone on the three Corvette forums. Bit the bullet at Christmas and replaced the Optispark. In my gallery you can see the pictures. The optispark itself was okay I believe but look at the contacts. They are so green with corrosion you can't even see the brass. Put it together and it ran awsome. Next week it was missing and stumbling. Much better than original as it had power but obviously not right as it was missing on the left bank. You could feel it at the exhaust. I measured the resistance on the injetors and one was an open circuit. I replaced them all today with a set of Bosch III's and it runs perfect. For years it has had a little stumble and that is completely gone. It idles smooth and runs fantastic. The values on the scan tool are down and everything looks pretty good.

Now, I'm not saying it is your issue as my car sat for 3 years and as I said has very low mileage so you can understand the corrosion but clearly those two things sorted out my car. The Opti-spark is an expensive one as the cap and rotor are $180 so I just bought the whole thing for $300 which included the cap and rotor. While you are there everyone recommends replacing the water pump which is another $180 which I am sure I did not need. All in all, it cost me $660 to get my car back to the best performance it has seen in years. I am not feeling too bad as other than oil changes... this is the first real repair on the car in 19 years.

Hope you get yours figred out soon. Summer is almost here.
Take care.
 

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