Another 94 miss/surge problem
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Another 94 miss/surge problem
I have a 94 LT1 that runs perfectly at idle and light throttle - even up to highway speeds. Under medium accel from around 50mph, I get a little preignition before it downshifts. Then, if I increase the throttle further, it will sometimes start to miss/surge immediately, and sometimes runs OK up to 4500 -5000 rpm before it misses. No codes, fuel pump pressure before starting is 42psi, drops to 38 after starting (seems within spec, but at the low end). I have recently replaced fuel filter, ICM and coil and fixed a vacuum leak - no change. Car has 140K - distributor, wires and plugs were replaced at 80K. I am thinking about a new fuel pump next, but I really have the feeling that it may be something simpler - maybe the EGR. Any thoughts, similar experiences - things to check?
Drifting
Joined: Sep 2003
Posts: 1,363
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From: 1994 LT1 Coupe 6-speed with FX3 & 2000 LS1 Vert 6-Speed with F45 Hunterdon County, NJ
Have you cleaned the Throttle-Body and Mass-Air-Flow (with products specific for each)? Either being dirty could cause the problem (had a stumble and miss at 1200 and 2000 rpm respectively on our 94 which went away after cleaning TB and MAF).
Also, what brand plugs and wires and what gap on plugs? If AC/Delco Platinum plugs they could have lost their 'pucks', so the gap is now too large. OE gap is 0.50, if gap was set larger could also cause the problem (i.e. NGK Iridium's not regapped -- which come gapped at 0.60 -- too large for our 94's ignition system).
Also, what brand plugs and wires and what gap on plugs? If AC/Delco Platinum plugs they could have lost their 'pucks', so the gap is now too large. OE gap is 0.50, if gap was set larger could also cause the problem (i.e. NGK Iridium's not regapped -- which come gapped at 0.60 -- too large for our 94's ignition system).
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Mike and Admiral, Thanks for the replies. I did clean the MAF and TB with correct cleaners - they were dirty, so I was optimistic, but no joy.
The plugs are Bosch platinum - it's been a while, but as I recall, they are the gapless variety. The plugs would be cheap enough to try, if a couple weren't such a pain to reach! The wires look to be OK and routed correctly - I will try the "dark night" test to be sure.
Wouldn't you expect the idle and low end performance to be rough if the problem is distributor/wires/plugs? When I say idles perfectly, I mean smooth as silk - and starts every time with one quick click of the starter motor.
Is there a way to diagnose the distributor without major surgery? I do have the Helms books, and basic talent to do the work as long as I can do it from the top side.
I have read some threads pointing at the FP relay - but doesn't that cease to be a player once the car is started?
The plugs are Bosch platinum - it's been a while, but as I recall, they are the gapless variety. The plugs would be cheap enough to try, if a couple weren't such a pain to reach! The wires look to be OK and routed correctly - I will try the "dark night" test to be sure.
Wouldn't you expect the idle and low end performance to be rough if the problem is distributor/wires/plugs? When I say idles perfectly, I mean smooth as silk - and starts every time with one quick click of the starter motor.
Is there a way to diagnose the distributor without major surgery? I do have the Helms books, and basic talent to do the work as long as I can do it from the top side.
I have read some threads pointing at the FP relay - but doesn't that cease to be a player once the car is started?
Racer
Joined: Jun 2007
Posts: 477
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From: Holbrook NY
I recently had a miss at 4500 to 6K rpms. I have 70K miles. I changed the plugs and wires. Miss is gone 
Plugs wore nicely, but were shot. Two of my wires had cracks. Runs real nice now. I thought I was due for an Opti...
Good Luck, Mike
Plugs wore nicely, but were shot. Two of my wires had cracks. Runs real nice now. I thought I was due for an Opti... Good Luck, Mike
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Instructor
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Thanks, I am becoming convinced that it is either opti, wires and/or plugs. I drove with the FP guage today, results were:
42psi at key on
38 at idle
35-38 at 1500 RPM
42 on acceleration
drops to 32 briefly when quickly releasing throttle
bottom line - when preignition and missing occurs, FP is 42PSI - I think that the fuel system is OK.
I did notice a little white residue at one place on the coil wire. I will try the "dark of night" test to see if I can observe a spark. Not to sound too cheap, but do any parts dealers sell just the coil wire without the whole set? (just for a quick try to either fix or eliminate that possibility?) If that does not fix it, I think that I will start at the plugs and work toward the distributor!
42psi at key on
38 at idle
35-38 at 1500 RPM
42 on acceleration
drops to 32 briefly when quickly releasing throttle
bottom line - when preignition and missing occurs, FP is 42PSI - I think that the fuel system is OK.
I did notice a little white residue at one place on the coil wire. I will try the "dark of night" test to see if I can observe a spark. Not to sound too cheap, but do any parts dealers sell just the coil wire without the whole set? (just for a quick try to either fix or eliminate that possibility?) If that does not fix it, I think that I will start at the plugs and work toward the distributor!
Le Mans Master
Joined: Jan 2006
Posts: 5,621
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From: Orlando FL
Thanks, I am becoming convinced that it is either opti, wires and/or plugs. I did notice a little white residue at one place on the coil wire. I will try the "dark of night" test to see if I can observe a spark.
Not to sound too cheap, but do any parts dealers sell just the coil wire without the whole set? (just for a quick try to either fix or eliminate that possibility?) If that does not fix it, I think that I will start at the plugs and work toward the distributor!
Not to sound too cheap, but do any parts dealers sell just the coil wire without the whole set? (just for a quick try to either fix or eliminate that possibility?) If that does not fix it, I think that I will start at the plugs and work toward the distributor!
When the opti went bad, that was the time to install new wires. My opti had similar symptoms as yours and I did check everything also but found nothing. I did change the coil so now I have a spare.
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Thread Starter
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Yes, GM does sell only the coil wire by itself. My wires insulation broke down at the water pump end and was arking to it. Cause a no start condition and a miss at times.
When the opti went bad, that was the time to install new wires. My opti had similar symptoms as yours and I did check everything also but found nothing. I did change the coil so now I have a spare.
When the opti went bad, that was the time to install new wires. My opti had similar symptoms as yours and I did check everything also but found nothing. I did change the coil so now I have a spare.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
Thanks. I checked last night, but did not see any visible arcing anywhere. If the problem is the distributor, wouldn't you expect to see poor idle and lower RPM miss, too? It is puzzling how smoothly it idles and runs up to 70mph under light throttle - will cruise all day averaging 21 MPG.
Yes, can have poor idle also as I did have. You have the same symptoms as I had. OK as light throttle but give it the gas and it bucked and coughed. Ran OK when cold on morning but once at temperature it starting acting up. Worse at times, in local traffic sometimes could not get going with the rest of the traffic.
These are classic opti symptoms but the wondering idle and jumping tach kinda thru me off. Just could not believe an opti could do all that, did not make sense or was logical. That’s why diagnosing an opti can be difficult because the symptoms and be wide and varying. Could not believe it was bad, tried to avoid it, but was the last stop and no place else to go. When I replaced the opti life was good again.
Last edited by pcolt94; Sep 29, 2008 at 12:24 PM.
Le Mans Master
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From: Wait 'til next year!!!!!! ---Emerald Coast of Florida
St. Jude Donor '10
My '94 had the same symptoms.. fell flat on her face under acceleration.... in my case.. new plugs and getting the injectors cleaned (and 1 replaced) did the trick.
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Thanks to all for the help. It turned out to be the plugs - actually probably just the one with a cracked insulator! Runs like a scalded cat again. The plug swap was easier than I had expected, once I removed the rear and middle fender well panels on the pass side and just the middle on the driver side. I was pleased to see that all plugs were a nice light tan color - no oil or carbon on any - not bad for 140K miles (only 60K on the Bosch plugs - I put AC OEM back in). I guess that I won't begrudge it the new coil and ICM that it didn't really need.
