I don’t know if this is the right place to post this as it doesn't concern a C4. As there are a lot of bright GM guys on here, here goes.

I have an 89 GMC Jimmy with a 4.3 in it. It normally runs smooth and has plenty of power. It has new plugs, wires, cap, rotor, O2 sensor and vacuum lines. Every once in a while it used to just miss a few times like when you step on a carbed car that had a flat accelerator pump, popping through the intake and then taking off. Recently it has become an everyday occurrence though. It idles fine. It cruises fine it runs great when you stomp your foot in to it but under medium throttle pressure, it pops through the intake but once again this is intermittent. It has not generated a code or failed completely. I have also noticed that it is becoming harder to start in the morning. Cranking longer than usual to fire off. Though the fuel pressure is 11 lbs. are these signs of a failing pump?

Any help is greatly appreciated.

 

Drivability problems are tough. My first thought was a dead spot in the TPS but you may be on to something with the fuel system. Is 11lbs fuel pressure acceptable for a TBI car?

 

I was told by a friend that it has 11 psi fuel pressure according to his gauge. The spec I read says it should have 10-12 psi. to me it sounds low for a fuel injector but, that's why I ask questions.

 

11psi is within spec. Replace the fuel filter then tape a fuel pressure guage to the winshield and read fuel pressure under load. Fuel pump may be weak or it just may be a plugged fuel filter

 

After you do the above, take a look at the EGR. I once had an EGR with a weak spring. The EGR is closed at idle and WOT, so it shouldn't affect those conditions. If the valve is opening too far at part throttle, it can lean the mixture and cause a lean misfire. Light throttle acceleration would likely be lean, too, like a bad accelerator pump on a carb. Good luck, and...

RACE ON!!!

PS. Who told you C4's don't have TBI?

 

First I found a correction for my pressure information. It should have 9-13 psi of fuel pressure according to the book. I should be OK there.

Second the EGR suggestion, I am going to test that nasty thing on the drive home. I am going to disconnect the vacuum line and see what happens.

Third I thought only 83 Corvettes had TBI. J/K I know they used TBI on 82-84. One of my neighbors had an 84 that ran great.

Thanks for all the help guys. I will report any success.

 

Kinda sounds like it to me.
One easy way to check the TPS. Get your self an analog multimeter, 20K ohm setting. Unplug the tps and measure across terminal 1 & 3. Then measure across 1 & 2 and slowly cycle through the throttle while watching the meter. You want a nice, smooth increase in ohms. No dips. Measure across 2 & 3 the same way.

 

As I have only a DMM with an analog bar, I used it to measure voltage frome pins 1-2 and 2-3 with the ignition on. Vehicle not running. It looked smooth but tough to tell with digital. The reason I discounted it is because sometimes it runs so nice like this morning on the way to work. I'd rather have it run better on the way home for a fast getaway. Thanks for the input! If the EGR doesn't test bad I may replace the TPS to be safe. They're cheap and easy to change.

 

There never was an '83 Corvette. 1982-1984 Camaros, Firebirds, and Corvettes, were available with the Crossfire, dual throttle body injection. In the case of the "F" bodied cars, it was an option on the 305. It was on a 350 engine and the the only thing available in the 1982 and 1984 Corvette. This is the infamous, L83 engine. The 1984 Corvette was the first C4 and the last L83.

RACE ON!!!

 

I was joking when I said 83.... My feeble attempt at humor.

J/K = Just Kidding

 

After looking at the EGR it operates normally. I did notice 2 things about the EGR though, it is a P.I.T.A. to remove and it has normal motion when vacuum is applied. I did disconnect the vacuum line and block it off at the valve. I didn't get any codes but the car runs so much better. There seems to be a lot less missing under load. My fuel pressure is still within spec. I did see one of the injectors appeared to leave large drops of fuel on the butterfly of the throttle body while the engine was on. I may have found another thing to look at.

 

Last Wednesday it DIED on the way to work and wouldn't restart. Rather than play with it in the cold and dark, I called AAA to tow me home. Besides this way I got to drive the Vette to work. Ok back to the Jimmy. Upon inspection this weekend, I found why it wouldn't start. One of the ECM fuses opened. After replacing the fuse it started up. It did have a HORRIBLE miss and NO POWER. I looked under the hood and found a shorted injector. It seems all of the OHMS leaked out of the injector's coil. Upon replacement it was like driving a less used car. It ran great so I filled the tank with gas and there was the miss I had originally started looking for. If I had hair I'd be pulling it out!

So after all of that, do ECM's ever become intermittent and not spit codes? What should the coil resistance of the injectors be? Could the ECM cause a miss in the 2000-2600 rpm range when it wants to?

 

hmmmmm

 

Check the hose that connects to the MAP sensor. In these old TBI cars it is very important that the vacuum signal is good to the MAP. In my old GM days I can tell you that I have found countless clogged or near plugged MAP lines. It isn't the hose necessarily but rather at the throttle body connection it gets all coked up with goo. You may have to remove the TB to clean out that port. Using a good strong carb cleaner like CRC or Berrymans and a small pick to dig out the deposits.

If that isn't it, you may have to find someone with a load bearing dyno to simulate the problem. With that he can load your truck just like its going down the road. The shop will have a ignition scope connected to the engine to observe any weird anomolies such as a bad wire, or a cracked spark plug. I know you replaced them all but that doesn't mean that a plug or a wire is not the culprit. You could even have an ignition module doing some weird things, or a ECM ground that is loose. So check all of your electrical connections, these things work on a 0 to 5v reference so grounds are very important. Another possibility is the pick up coil inside the distributor, these have been a known cause of drivability problems as well.

 

Thanks Thomas,

I think there will be no more troubleshooting. I will clean and replace the vacuum port and hose. If that makes no change I will change the MAP sensor, the ignition pasrts and if all else fails, the ECM. It is getting to the point that the frustration of it kicking my butt will be worth the money spent to get it fixed.

Thanks again for the input.

 

Don't just throw money at it. All of those items can be TESTED. Find and fix the PROBLEM. The "shotgun" approach can get expensive, and be frustrating.

RACE ON!!!

 

I agree the shotgun method isn't the best route. I have spent so much time on it as it is, I am wondering what my time is worth. Seems if I were making $10 an hour, I could have bought a couple hundred bucks worth of parts already. Most parts for this POS are fairly cheap. With so many miles on it, most iof them are going to break soon. Is that poor justification for laziness?

You are right I am going to find the problem part and replace it. Then SMASH that part with a hammer unless there is a core charge.

 

and you need to KNOW what it was

 

I know it's been said. I'm not sur eif/how you tested it. But The EGR system in these vehicles SUCK. We had a ton of them in the military & ALWAYS had to keep replacing the EGR valves, because of what you just described to a T.
I'm not sure how you drive it. But on base they dont go fast & build up alot of carbon. Then they malfunction.
Just my experience with that.

 

My 89 Jimmy now runs great. Thanks to all for the help. Here is what I replaced and where the real problem was.

BOTH fuel injectors were replaced though only 1 was shorted.
One ECM fuse because the silly thing decided to open.
Ignition module was replaced due to ease and fluctuating timing.
Distributor Pickup coil was replaced due to low resistance.
Fuel pump was replaced due to low flow. It was caused by a clogged sock. It was so much work to replace the sock, I didn't want to put an old pump back in.


At this point it ran so much better but had a little stumble. I decided to re-test the plug wires and found a hard spot in the middle of one even though the resistance was within spec.

The real problem was one injector and the plug wires as they made the biggest difference in performance.

Thanks again.