Corvette1991

I am new to this forum and new to working on any Corvette but I have a good bit of experience working on many different makes and models of domestic and import cars. A friend of mine recently purchased a 1991 Corvette that has 10,600 miles on the clock. The car appears completely original and unmolested with the exception of the factory head unit that has been replaced with a late model cd player. It is all numbers matching, clean carfax, stock L98 with aluminum heads.

The owner wanted me to look into a few issues he discovered after taking his new car on it's first couple of rides:

- The engine will stumble at idle and during light acceleration up to around 40mph. After driving it myself, it feels like a misfire and the engine stalled on me after coming to a stop light a couple of times after the idle had dropped below 500rpm.

As an initial start to diagnosis I inspected the spark plugs which turned out to still be the originals from 1991. The electrode ends were in very good shape with minimal erosion and the gaps ranged from .038" to .042" not far off of the stock .035". The insulator ends of the spark plugs were showing signs of atmospheric corrosion with the hex and exposed steel portion being rusted. At the cost of approximately $3 a piece, I chose to replace all of the plugs with equivalent new AC Delco versions. The misfire continued to persist. I jumped online to check into any forums or websites that may point me in the direction of a likely culprit.

- I checked the injector resistance across all 8 injectors but only cold, I didn't have time to check them with the engine hot. All injectors measured 16.5Ω to 16.1Ω.

- The next part I would like to proactively replace would be the fuel filter. I do not know what the state of the fuel was prior to the new owner taking delivery (stabilizer etc.??). So I have to assume the worst case scenario in that the car sat for most of its 23 years not being driven and definately not being at least maintained for its natural aging over time instead of mileage.

My question is other than replacing the cap/rotor and plug wires after the fuel filter if the misfire still continues, where would be another place to check? I am going to check the injectors after warming up the engine to verify that the resistance remains the same as when cold.

I know that the EGR valve can cause misfire/ rough idling if it is stuck open but I've read that the 1991 EGR valve rarely suffers from issue unless an aftermarket exhaust system is installed that doesn't allow enough back pressure to allow normal operation. Also I've read that a code 32 should be present if the EGR is at fault, there are no codes. The exhaust system on the car is completely stock including all catalytic convertors. My last thing on the list would be to check operation of the EGR but since so little miles are on the car I would expect it to be in good operating condition. Any suggestions are appreciated.

GKK

Unfortunately, any 23 year old vehicle with only 10k miles, will have issues from just sitting...

First, Drain the old fuel out and fill with clean fuel. Change the fuel filter also.

Check the IAC valve, (Idle Air Control valve). It's screwed into the passenger side of the throttle body and adds extra air into the plenum at idle. The IAC valve plunger could be frozen and not opening and closing correctly, (do not try to push the plunger in because it works on a worm gear and will break if pushed down). Just check the tip and chamber for carbon deposits and clean with carb cleaner but, do not spray into the IAC valve directly.

If the IAC valve is OK, check the Fuel Pressure. Connect a fuel pressure gauge to the passenger side fuel rail schrader valve and see what the fuel pressure is. Fuel pressure should read 37-42 psi with the engine on and vacuum hose removed from the fuel pressure regulator.

If the fuel pressure is low, replace the fuel pump which is easily accessible from the top of the fuel tank.

If the fuel pressure is good, I would suspect that the Fuel Injectors are clogged or leaking. The stock Multec fuel injectors are not compatible with today's fuel which contains alcohol and will eventually short the internal coil insulation. Replacement Bosch III injectors are available from forum member Jon Banner at http://fuelinjectorconnection.com/sh...?productid=194




.

Corvette1991

GKK, Thank you for your helpful input. My background in automotive repair is mostly with Acura/Honda due to being a service technician for 6 years at a Honda dealer. Being so the IAC is a familiar item to check for a poor or surging idle on those cars as well.

On your recommendation for replacement Bosch injectors, if the originals are indeed needing replacement, are the Bosch injectors compatible with Ethanol based fuel?

I will include your tests to the fuel pressure after replacing the fuel filter to verify the pump is working correctly.

I still have the gouge marks on both my left wrist and top of my right hand as evidence to doing the passenger side spark plugs this last weekend...lol. I had removed the A/C to exhaust manifold brace and moved a couple of heater hoses and air hoses but damned if I had enough room to move my fat hands around in there...

Thanks again for the suggestions. If you or anyone thinks of anything else feel free to add. I will reply back with what I find this weekend when I can get to the car again.

GKK

Yes, the Bosch III fuel injectors are modified to fit the 88-91 C4 Vettes and are Ethanol compatible.

Also, if you remove the plastic inner wheel well panel above the shock, (4 bolts) access to the passenger side spark plugs is a lot easier!...

Corvette1991

Ok, I was able to do some more work on it this weekend for my friend.

I checked the fuel pressure, it was spot on for where it should be roughly 39 psi. I went ahead and purchased the fuel filter and spark plug wires, distributor cap and rotor with the intention of replacing all of the ignition components in the hope that it would either take care of the misfire or at least narrow the possible problem down.

I found when replacing the distributor cap that something had been done to it at one point in it's life as some of the connections on top of the cap were bent slightly. I also found around the passenger side back valve cover where the wires have wire loom around them I'm guessing for protection from the heat of the EGR pipe and header the wires were zip-tied together. All of the other spark plug wire separator/connectors were there including one behind the distributor for the 3 long wires, 1,2 and 3 which cross behind the distributor.

The distributor cap had what looked like normal wear and tear with slight corrosion on the contacts but otherwise looked ok. The wires appeared in ok condition.

The misfire/ rough idle still persists however and after relaying this information back to the owner I told him that I am basically narrowing the items down to the fuel injectors being bad. I was told that the fuel in the tank is new but more than likely the fuel has contained ethanol since its introduction into gas stations because the previous owner and new owner did not know about the problem with ethanol containing fuel and the multec injectors.

My next course of action is to replace the fuel filter even though the fuel pressure looked ok just to know that it is good. Then I'm going to check the EGR valve for operation being ok and the IAC. If everything checks out ok the last thing I'm going to do is drive the car around, get it nice and hot and check the injector resistance again.

Can the injectors still be faulty and not show a lower resistance when hot? I would just like to know for sure that we have an issue with them before I tell the owner that he should replace them. Everything up to this point can be chalked up to a general tune-up or wear items but the injectors is quite a bit more labor time and monetary investment to just replace for the sake of replacing them. It's one of those things where the inspection almost warrants replacement because of the amount of work it takes to remove and inspect them for leaks/ damaged O-rings.

GKK

If the injectors measure 16 ohms but, the engine still runs rough the injectors could be clogged.

The Multec fuel injectors cannot be cleaned like the older Bosch injectors so, replacement with the more modern Bosch III fuel injectors is the best option.

Also, the TPI engines Throttle body, Plenum and Runner bolts can become loose over time and cause a vacuum leak.

Corvette1991

I received an update from the owner today. He is obviously not happy that his new purchase which was advertised to him in very well tiptop shape. Looking at the car you wouldn't expect it to have any issues as it is super clean.

The owner explained the situation he finds himself in with the car not running correctly/ misfiring and wanted to know if the previous owner had noticed or had any records to reflect any maintenance being done on the car after what I had found so far.

He was sent an inspection receipt from June of 2010 that showed the car had 9,994 miles on it and other than brake and tire measurements shows little else.

A receipt from June of 2011 that showed the car had 10,170 miles on it for an inspection, replacement of the fuel filter, air filter, serpentine belt, and oil and filter change.

The last receipt is from July of 2012 and is for an inspection, 8 fuel injectors, a distributor pick up coil and 3 hours of labor.

So I now know why I found the distributor cap terminals bent. The "replacement" injectors installed in 2012 appear to be the original style Multecs from all visual observation, I have not removed them yet. I would not think that a pick up coil would have gone bad on a car with so little miles on it but I suppose due to the age it could be possible.

I am still going to replace the fuel filter with the AC Delco version this coming weekend and then check the other sensors.

Since the car has mostly sat still for the last 4 years, it appears safe to assume that even with the new injectors installed in 2012, if the fuel going through them had ethanol in it that the fuel injectors could be at fault again especially if they were replaced with the same kind that came out, the Multecs.

playsdixie

as GKK has said...check your Iac.....sounds like that could be your problem...a lot easier that tearing it apart to get to injectors. You haven't mentioned checking it....

hooked073

just for laughs and giggles check the timing. You will need to unhook the est wire. It is on the left had side around the rear of the valve cover area. it is a tan wire with a black tracer.

Corvette1991

The owner of the vehicle without my direct recommendation wanted to replace the injectors with Bosch III's regardless of the condition of the fairly new Multec injectors that he has a receipt that showed replacement in 2012.

Today I first replaced the fuel filter as a precaution and to rule out its condition as any contributing factor.

The owner purchased Bosch III injectors from John Banner at FIC. He also purchased the intake plenum gaskets as I had recommended him to. I cleaned the fuel rail out very well and installed all 8 new injectors and all new intake plenum gaskets. On startup, we notice the problem is still present.

I checked the IAC and it is clean and operating fine.

The last thing I did before calling it a night in frustration and dwindling daylight and increasing mosquitos was try to check the timing to see if it was where it should be per hooked073.

Spec is 6° BTDC.

I disconnected the EST wire that as factory taped to the main wiring harness by the brake booster.

We then started the engine and immediately noticed it was not misfiring with the EST wire disconnected.

I checked the timing with a timing light to find that the mark was fairly far off of what the spec was. On the scale next to the crank pulley are marks that read in even numbers and the mark was toward the top of those marks almost above them all together so in the neighborhood of 12°+ instead of the 6° it is supposed to be. I intend on adjusting it to the factory position tomorrow morning.

Is it possible that the ignition timing being so far advanced it would cause it to misfire?

Corvette1991

I just did a search and found this:

http://forums.corvetteforum.com/c4-t...lugged-in.html

Also this information coupled with the fact that there is a receipt showing that the pickup was replaced at some point in the past has me thinking that either the gap between the reluctor ring and magnetic pickup coil was never set when the pickup was replaced and/or the timing being so far advanced is causing the misfire. I will get to the bottom of both in the AM.