Need help diagnosing a problem on 1994 Silverado

ThomasV · October 31, 2006

I'm having some trouble diagnosing a problem on my 1994 Silverado. About a month ago I took it into the dealer because at idle it would begin stumbling / missing sometimes - but not all the time. It had gotten progressively worse to where it did it on every drive. Typically, it would only do it after being warmed up and when I'd come to a stop. It would idle perfect and then all of the sudden it would start missing to where the whole truck would shake. I would normally put it in neutral and rev it and it would go away. I'd take off driving and everything would be fine - until the next stop or maybe even a few stops later.

The dealer did diagnostic's and checked fuel pressure, ignition, sensors, emissions, etc. and everything checked out good. They said they couldn't get it to miss, which is hard to believe because it was doing it when I pulled into the service lane. Anyway, they suggested flushing the injectors, which I agreed to. That fixed it - for 3 weeks, and I drove it everyday. Now it's creeping back again. I can feel the same thing happening again, although the miss isn't as bad yet.

Yesterday, I put new injectors in. It does drive and idle smoother, but the miss is still there. I drove it to work today (about 8 miles) and it ran perfect all the way here. As soon as I pulled into my parking spot, it started missing.

Ok, so the fact that the injector flush fixed the problem for 3 weeks is interesting. I'm not sure where in the fuel system the dealer connects the flush kit to. Could it be a problem with the fuel pressure regulator in the TBI?

Within the last year it has had new wires, plugs, cap, rotor, fuel filter. I put a new GM EGR on about 3 years ago.

I'm looking for suggestions / advice. Would you take it back to the dealer with the above information? It cost me $170 the first visit to the dealer for diagnostics and the injector flush. I thought they fixed it - until last weekend.

HELP!!!!

RCH · October 31, 2006

I got some bad gasoline once in my Safari, sounded a bit like what you are experiencing. It ran great on the Highway, but when I pulled off it would sputter and miss badly. We were on our way home on vacation, when we got home I installed a new fuel pump, fuel filter, and screen, ran fine after that. Maybe check the quality of the fuel.

ThomasV · October 31, 2006

I got some bad gasoline once in my Safari, sounded a bit like what you are experiencing. It ran great on the Highway, but when I pulled off it would sputter and miss badly. We were on our way home on vacation, when we got home I installed a new fuel pump, fuel filter, and screen, ran fine after that. Maybe check the quality of the fuel.

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I was wondering about that myself. I put some Sea Foam in the tank last night and will fill up tonight - at a different station.

Thanks for the reply.

bobcat5544 · October 31, 2006

With out having a scanner hooked up to it to monitor the engine it's to figure your problem out.

But you can pull the spark plugs out and look at them that will help deturmain if it is a fuel or spark problem. if you can take pictures of the plugs and post them.

Also what type of plugs did you install.

ThomasV · October 31, 2006

With out having a scanner hooked up to it to monitor the engine it's to figure your problem out.

But you can pull the spark plugs out and look at them that will help deturmain if it is a fuel or spark problem. if you can take pictures of the plugs and post them.

Also what type of plugs did you install.

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I replaced the plugs with OEM AC/Delco.

RCH · October 31, 2006

[The dealer did diagnostic's and checked fuel pressure, ignition, sensors, emissions, etc. and everything checked out good. They said they couldn't get it to miss, which is hard to believe because it was doing it when I pulled into the service lane. Anyway, they suggested flushing the injectors, which I agreed to. That fixed it - for 3 weeks, and I drove it everyday. Now it's creeping back again. I can feel the same thing happening again, although the miss isn't as bad yet.]

He had the dealer scan it.

95Sierra2500 · November 1, 2006

Definately sounds like your problem is coming from the fuel you're using. Perhaps the station you're getting fuel from has dirty or bad gasoline? They may have a tank that rust is getting into, and contaminating the gas, or something along those lines...Sometimes getting what's cheap isn't the best idea.

ThomasV · November 1, 2006

Definately sounds like your problem is coming from the fuel you're using. Perhaps the station you're getting fuel from has dirty or bad gasoline? They may have a tank that rust is getting into, and contaminating the gas, or something along those lines...Sometimes getting what's cheap isn't the best idea.

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I use nothing but Texaco gas.

BIGNN · November 1, 2006

i would flush out your gas tank and get a locking gas cap.

bobcat5544 · November 1, 2006

Why would you waste time and flush out a gas tank, when there is no proff that the gas is bad.

At this time there is no way to deturmain if it is fuel, eletcrical, or mechanical. with out inspecting and testing things

bobcat5544 · November 1, 2006

Definately sounds like your problem is coming from the fuel you're using. Perhaps the station you're getting fuel from has dirty or bad gasoline? They may have a tank that rust is getting into, and contaminating the gas, or something along those lines...Sometimes getting what's cheap isn't the best idea.

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I use nothing but Texaco gas.

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What engine do you have

ThomasV · November 2, 2006

Definately sounds like your problem is coming from the fuel you're using. Perhaps the station you're getting fuel from has dirty or bad gasoline? They may have a tank that rust is getting into, and contaminating the gas, or something along those lines...Sometimes getting what's cheap isn't the best idea.

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I use nothing but Texaco gas.

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What engine do you have

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Well, I think I found the problem. I decided to start with the easy things. I bought a new PCV, fuel filter and Techron fuel system cleaner. I started with the PCV and went to pull the vacuum hose off and it like fell out of the right angle rubber connector fitting on the bottom of the TBI. The right angle connector had a big crack on both ends and was obviously causing a vacuum leak. As a temporary fix, I wrapped both ends tightly with black electrical tape. I did a high-tech suction test (with my mouth and finger, lol) and the tape is working for now.

I also did a general check of the rest of the vacuum hoses. The rest were still good.

I then decided to check the IAC. I went to remove the IAC and discovered it wasn't installed very tightly. I removed it and it looked super clean, so I just re-installed it, making sure it was installed tight.

When it was at the dealer, they charged me for a new fuel filter. I checked it and they did in fact install a new one. I pulled it and drained it into a clean white cup and also sprayed some carb cleaner into it. The gas was clean with no signs of any contamination, so I just re-installed it.

I took it out for a long drive with several stops and it didn't miss once at idle. I think it was the vacuum leak. I'll replace the right angle vacuum hose connector and see if it continues to run well. So far, it's running like a new truck again!

At this point I'm thinking that while it was at the dealer and they were running the tests that maybe they pushed the vacuum hoses back on and it was fine for a few weeks and then slowly the hoses backed off or something. I'm a little pissed that they didn't catch something so simple (if in fact time proves that it was the vacuum leak causing the problem).

Thanks to all for your suggestions. This truck is my sole means of transportation, so when it's not running right, it kind of puts a damper on your life as you're hesitant about driving it. I have to say that keeping in the right frame of mind and tackling the problem as a challenge proved a much better approach than getting pissed, lol.

SPARK CHASER · November 2, 2006

I had a similar idling problem on my 1993 silverado. I too found the IAC loose on the throttle body. I tightened it up and the slight idle "stumble" went away. Your PCV hose may or may not have been part of the problem. I found my PCV connector cracked at the throttle body several years ago, but it didn't cause an idle problem on my truck, just a whistling sound.

ThomasV · November 3, 2006

I had a similar idling problem on my 1993 silverado. I too found the IAC loose on the throttle body. I tightened it up and the slight idle "stumble" went away. Your PCV hose may or may not have been part of the problem. I found my PCV connector cracked at the throttle body several years ago, but it didn't cause an idle problem on my truck, just a whistling sound.

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Thanks spark chaser. It was either the loose IAC or PCV vacuum leak. It's been a few days now and it's still running great. With the new injectors and above fixes, it's running soooo smooth and feels like it has so much power than before.