Problem with '94 Chevy 4x4 pickup

[parkertown]

The truck has the 4.3 V6 and auto trans. The problem occurs when the truck has been sitting for more than a day or two. The first time it is driven, the truck starts to backfire and run very rough after it has warmed up. It usually cranks fine. The backfiring will continue for as long as I drive the truck during that first drive. After parking the truck for a few hours, it will fire right up and run like new. If I continue to drive it daily, it will run perfect every day. Only when I go a couple of days without driving it do the symptoms reappear. The truck has a new fuel filter, battery, alternator, and starter. This is a very low mileage vehicle for it's age with only 47000 miles on it.

Does anyone have any idea what the problem could be? I think it has to be fuel related and have tried Techron additives that only slightly improve the problem. Thanks.

 

[MrCritical]

Check engine soon light on?

 

[parkertown]

The yellow "Service Engine Soon" light does come on intermittently
but never when the truck is driving poorly. A dealership mechanic didn't think the problem was related to this, but I thought it might be. I believe he said there was a tube loose on the egr valve and he reconnected it. That was about a month ago but didn't solve the problem.

 

[NYEngineer]

How old are your cap, rotor and wires? Your coil? Maybe during that first drive, you're cooking moisture out of your wires or a cracked coil. It's fine once it dries out, but absorbs moisture again as it cools off.

 

[Islandvic]

Try getting the stored code using the paper clip in the data port method and the stored code will blink the "check engine light" in a sequence. Google the sequence and it will tell you the stored code.

You may need to replace the EGR. I would change out the EGR valve with a AC Delco unit.

Good luck

 

[parkertown]

Thanks for the replies. The dealer checked for codes and only the egr problem showed up. They thought they fixed that by replacing the tube or wire, but the service engine soon light still reappears for about 10 minutes every time I drive the truck, then goes out on it's own. The plugs, wires, and distributor cap are all original (14 years old!) but with only 47,000 miles. Dealer said if it were plugs or wires it would run rough all the time, not just the first time I drive it after sitting a few days. I think NYEngineer may be on to something with the distributor cap.

My dealer service guy is very "old school" and doesn't think the egr valve would cause the drivability problems I'm having. He even said that when you get a red light on the dash, bring it in for service but a yellow light is nothing to worry about as it's "only" emissions related.

 

[JTK]

Being pre OBDII (a great thing BTW!), the CEL or codes wont tell you all that much. I'm thinking you are definitely on to something with a new dist cap and ignition wires. 14yrs is waaaay too long for those items. I remember changing out the cap on my dad's 1990 C1500 W/T 4.3L 3-4x in the years he owned it. It would rear it's head after a cold winter spell on the first warmer/damp day.

Joel

 

[Islandvic]

I have a 1995 GMC K1500 that started having mild symtoms then became worse. It had bad idle issues and the truck would lose power then surge when driving among other things.

I first replaced the cap and rotor, wires and plugs, fuel and air filter, PCV valve etc etc. It got a complete tune up and it didnt help at all.

Then a guy at work I was talking to had the exact year model and make as mine and had the same issues.

He replaced the EGR valve, IAC solenoid and MAP sensor, all with AC Delco parts. Problem solved.

I did the same thing, then my problems were solved! It drives smooth now with no issues.

I have researched older GM's and EGR's and supposedly they clog up more often than most vehicles.

If your mechanic hinted toward the EGR valve, REPLACE it! It's cheap. You can get an AC Delco from http://www.rockauto.com for the same price as a generic from the local parts store.

Also, check your ignition timing!

Good Luck!

 

[pair of Blazers]

sound like the classic CPI FPR leaking to me.
assuming its a CPI engine VIN W in the eighth place. I would start w/tune as stated above also read this thread on SSF but you'll have to register to read it. there is a lot more info there on the 4.3

 

[parkertown]

Replaced plugs, wires, cap and rotor today. NAPA had them all in stock. Will have to order new EGR valve, so I decided to try the tune up first to see if it fixes or improves the problem. It couldn't hurt, because the old dist cap was bad and the plugs were slightly fouled. If the problem continues, EGR valve will be next.

 

[Headnsouth]

Maybe a hung up lifter? My 97 Saturn if let sit for a week or so would startup and running like heck till it refilled. Since I've used auto-rx it hasn't returned.

 

[parkertown]

Update:
The new dist cap, rotor, wires and plugs appear to have solved the problem. The vehicle sat for several days after the tune up in flooding rain and fired right up when I went to crank it. It hasn't had the slightest knock, miss, or backfire since and is running like new.

Thanks to all for their input on this issue.