Help! 1994 misses and backfires at all speeds

[strayrazor]

I was running 87 octane in it and for awhile had been having a problem when pulling in higher gears, more noticable when warm. I picked up some Gumout injector cleaner, hoping if it was water in the tank that'd dry it, then I went and filled up with a tank of Chevron 93 octane, containing 10% ethanol. The engine started missing worse and backfiring a bit immediately after. Fortunately, I was only a few miles from home, the symptoms seemed like a fouled plug or bad wire from past "ordinary vehicle" experience. I hesitate to start working on this complex piece of engineering without a good consult here first. I appreciate any help

 

[Towu4free]

I was running 87 octane in it and for awhile had been having a problem when pulling in higher gears, more noticable when warm. I picked up some Gumout injector cleaner, hoping if it was water in the tank that'd dry it, then I went and filled up with a tank of Chevron 93 octane, containing 10% ethanol. The engine started missing worse and backfiring a bit immediately after. Fortunately, I was only a few miles from home, the symptoms seemed like a fouled plug or bad wire from past "ordinary vehicle" experience. I hesitate to start working on this complex piece of engineering without a good consult here first. I appreciate any help

i had a similar problem and cleaned the throttlebody,MAF and run some BG through the tank and it cleared it up...i did however find a nasty plug

just my .2 cents keep posted

 

[ZumZum]

You haven't recently washed the motor or otherwise got it wet somehow have you? It sounds like it could possibly be the dreaded opti-spark.

How many miles on the car?

 

[Toms007]

You haven't recently washed the motor or otherwise got it wet somehow have you? It sounds like it could possibly be the dreaded opti-spark.

How many miles on the car?

Sounds like what I was thinking.....Does it have any codes set?

 

[strayrazor]

I've never wet the engine, but I've only owned it for 8 months, could have some past corrosion I suppose. Hasn't thrown any codes. Have my doubts about the opti because it worsened suddenly after the increased octane and fuel injector cleaner. Due to that I suspect Oxy Sensor. I talked to a Chevy dealer tech who does our Tahoe work, and he said it's difficult in this old a model to get an accurate scan, but he'll take a look see at it in the next few days. I'll keep y'all posted. And Thanks!

 

[strayrazor]

As to the miles, about 66K

 

[Paul Higg]

The Gumout may have damaged the injectors. I use just a small amount of Lucas. I read that the injectors can get damaged easily. I forget what the ohm value of the injectors is. I can look it up. I would ohm the injectors, check for water in the fuel and gently clean the MAF. I would do as much as I could without tools.

Since it is not throwing any codes it could also be an off-idle bog. Like Towu said it may be a very dirty throttle body. I overhauled mine last summer and it was filthy. When I was done I punched it and left rubber! That was a huge improvement.

Do you have an FSM? If not I can help.

Paul

 

[Paul Higg]

Oh, turn off all the lights and run it and look for spark jump, you may have a bad wire or an improperly seated plug wire.

I doubt very much it was the ethanol or low octane as I and many others run 87 a lot of the time. My best guess would be water in the gas. There have been a lot of unscrupulous people putting water in the tanks to scam people lately. By the way, I live in Douglasville!

Good luck,

Paul

 

[Paul Higg]

One LAST thing, welcome to the CAC!!!! Look for Junk to chime in, he's one of the best! otfl

 

[strayrazor]

Thanks, Paul. Maybe we'll meet up at an event in the future. I read where the pre 92 models were vulnerable to injector cleaner, something about the o-ring composition, 94's aren't supposed to be affected, and it was immediate, so I don't know if they would have deteriorated that quickly. That said, watch me be wrong! BTW, What's an FSM?

 

[ZumZum]

BTW, What's an FSM?

Factory Service Manual. They're a valuable tool for trouble shooting. You can usually find them on Ebay or the likes.

 

[strayrazor]

Thanks everyone for the input. I'm in a time bind, so it'll have to go to Matt, our Chevy tech. I will however post pronto when I have more info. And I'll try to get a FSM, now that I know what it is, LOL. These acronyms, gotta be a youngster sometimes to keep up...Duh for me

 

[John Robinson]

Gas

Have you run the tank out that you bought when it started to run badly?

If you still have the gas that you put into the car when it went bad you are probably suffering from bad gas. Yes gas can be bad and cause all sorts of problems. You might not need to worry about the opti or other parts if it is bad gas. Just run it and when the tank gets down add fresh gas. After a couple of half tanks of fresh gas it may start to run better. Another indicator of bad gas is that the mileage drops way off.

I have seen tanks of bad gas that have had as much as a gallon of water in them. When its that bad the engine is toast in the first few blocks from the station.

Welcome to the CAC and keep us updated so we can all learn from your experience.

I have just reread your thread. I think you have two non related things going on here. First the problem with the performance when running 87 octane fuel. You have a 6 speed which means that you are really lugging the engine in 6th gear at modest speeds. What this means when you are using low octane fuel is that you have forced the ECM (Engine Control Module "Computer") to retard the spark to prevent pre ignition on the lower octane fuel. Yes it will run this way but if you are trying to accelerate in the higher overdrive gears it will cause the car to not pull cleanly. You might try shifting down a couple of gears to pass or accelerate which raises the RPM's and reduces the load on the engine.

Problem two is not related to problem one it is what I eluded to earlier in this reply and that is a bad tank of gas.

 

[Paul Higg]

Hey, very good John. Very informative post. Another keeper!

Paul

 

[strayrazor]

Thanks all. Out of curiosity, I pulled the passenger side #1 plug wire and to my surprise , no spark. Not to jump to a conclusion about bad Opti, I pulled wire #2 and put it in #1's position in the Opti and spark plug, and voila, sparky and strong, albeit engine was barely running on 6 cyl. Going to get a set of wires this afternoon and report later. Keeping my fingers crossed, and Matt on hold

 

[Hib Halverson]

There might be some incorrect assumptions in this thread. Perhaps I can help clear them up.

First of all, there are a great number of Gumout-branded products but only one claims to absorb water from gasoline and that's "Regane® Complete Fuel System Cleaner". If it doesn't say that on the label, it's not going to help with a water problem. It is possible that is the problem but, given the symptoms probably, not likely, IMO. Nevertheless, to absorb water from gas you need one of two products which will work better than Gumout, either "Red Line Fuel System Water Remover and Antifreeze" or "Chrsty Dry Gas".

As for 87 octane...a 94 LT1 will run on 87, just not as well as it will run on 91. The post above that suggests that when running on 87 forces the ECM to retard spark. That's kinda-sorta true. What really happens is that use of 87 octane may make the engine more prone to detonate. In 6th gear, the engine is no more prone to detonate than it is in other gears. Regardless of the gear selection, if right combination of low octane, engine speed, load and temperature are present; the engine detonates. When it does that, the knock sensor picks up the noise and the ECM, once it receives a knock signal, will retard the spark slightly until the detonation stops then it dials the base timing back in. If this happens at low engine speed at high load, the engine will not miss, but it just won't pull as well as it would without detonation-induced spark retard.

I think you are on the right track as far as checking out ignition issues. I'd be looking at the plugs, the wires and the cap on the distributor.

Good luck.

 

[Paul Higg]

Good troubleshooting!

 

[strayrazor]

OK, here's the update. I was wrong on the wire diagnosis. Double checked and had been looking at spark from another wire. Anyway recheck with a new wire and #1 still wasn't firing. Nursed the car to Matt's and he just called with the bad news. It's running on 5 cylinders, he was surprised I made it 25 miles to his garage! The distributor needs to be replaced, and while he's at it we're gonna replace the water pump, too, being a it comes off during the proceedure anyway. this is going to be about a 1K by the time all's done, and that's using GM parts, which Matt advised. BTW, he said the new distributor would be of the later design, vented and sealed. Still looks like we'll sell 'er this spring and save up to buy a later model. Thanks all for the input!!

 

[Towu4free]

OK, here's the update. I was wrong on the wire diagnosis. Double checked and had been looking at spark from another wire. Anyway recheck with a new wire and #1 still wasn't firing. Nursed the car to Matt's and he just called with the bad news. It's running on 5 cylinders, he was surprised I made it 25 miles to his garage! The distributor needs to be replaced, and while he's at it we're gonna replace the water pump, too, being a it comes off during the proceedure anyway. this is going to be about a 1K by the time all's done, and that's using GM parts, which Matt advised. BTW, he said the new distributor would be of the later design, vented and sealed. Still looks like we'll sell 'er this spring and save up to buy a later model. Thanks all for the input!!

when i was having problems with mine i was hoping and praying it wasnt the opti and it turned out to not be i did a search on here and it seemed that alot of people have replaced them when they didnt need to i think JUNK was the guy who said something about if he had $1 for every opti replaced that was not bad hed have another vette or somethin like that anyway just double check and good luck you man!!!

P.S. mine is identical to yours except its and Auto...any idea how much you may want for it?lol my buddy is lookin for one happy vetting!!

 

[Toms007]

OK, here's the update. I was wrong on the wire diagnosis. Double checked and had been looking at spark from another wire. Anyway recheck with a new wire and #1 still wasn't firing. Nursed the car to Matt's and he just called with the bad news. It's running on 5 cylinders, he was surprised I made it 25 miles to his garage! The distributor needs to be replaced, and while he's at it we're gonna replace the water pump, too, being a it comes off during the proceedure anyway. this is going to be about a 1K by the time all's done, and that's using GM parts, which Matt advised. BTW, he said the new distributor would be of the later design, vented and sealed. Still looks like we'll sell 'er this spring and save up to buy a later model. Thanks all for the input!!

OK, I think you may be judging the car too harshly. Optis go bad, that doesn't mean that the car is a hunk of junk. I understand your frustration I went through it too. Check this thread.

http://www.corvetteactioncenter.com/forums/c4-technical-performance/106269-sort-stumble-idle.html

But getting rid of it is being a bit rash. Once it is fixed you should have yourself a nice car. I love my 94, and as frustrated as I got with it I didn't feel the need to ditch it.

Now on the "new style opti" without changing out the cam/cam cover I don't believe that you can use the new style. (I could be wrong.....but I doubt seriously that I am). That is one of the reasons that I went with an MSD unit vs GM. And besides the GM opti that went bad had only 6k on it.

BTW if you are still set on selling that POS after you get it fixed, I'll give you $1000.