LT1 stumbles under high load
Thread Starter
Instructor
Joined: May 2003
Posts: 162
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From: fort mill sc
LT1 stumbles under high load
My '95 LT1 stumbles under high load (full throttle) usually in the 4600 to 5500 rpm range. There is also a lean popping noise in the exhaust (light backfire). All other engine performance is normal. Problem is only at full throttle. I have repalced the fuel pump and strainer and it did not fix the proble. It seems like a fuel delivery problem. I am replacing the fuel filter next. I measured fuel system pressure at the schrader valve on fuel inlet pipe and it's about 45 psig and drops to about 42 when stumbling occurs. Any suggestions on problem?
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
Re: LT1 stumbles under high load (rmd8136)
Plugs not firing. Check the resistance of the plug wires, they should measure about 5000 ohms per foot. If you have plugs with a lot of miles on them, install new ones.
Race Director
Joined: Feb 2002
Posts: 11,268
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From: Honolulu, HI
Re: LT1 stumbles under high load (rmd8136)
How do I fix it?
Also, before you replace your opti, do plugs, wires, and do more research before you do the opti because it's a big job.
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Team Owner
Joined: Aug 2000
Posts: 40,156
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From: San Pedro CA
Re: LT1 stumbles under high load (rmd8136)
Opti-Spark.
It is the distributor... GM put it on the front of the engine... below and kinda under the waterpump, and above and behind the harmonic balancer.
if water is not its friend, antifreeze is it's enemy.
they get wet inside, it builds up, causes carbon tracing and eats bearings.
not good.
they cost upwards of 1200.00 if you have a dealer do it!!! 1/2 that if you do it... the job is not hard, just time consuming.
It is the distributor... GM put it on the front of the engine... below and kinda under the waterpump, and above and behind the harmonic balancer.
if water is not its friend, antifreeze is it's enemy.
they get wet inside, it builds up, causes carbon tracing and eats bearings.
not good.
they cost upwards of 1200.00 if you have a dealer do it!!! 1/2 that if you do it... the job is not hard, just time consuming.
Race Director
Joined: Feb 2002
Posts: 11,268
Likes: 14
From: Honolulu, HI
Re: LT1 stumbles under high load (bogus)
they cost upwards of 1200.00 if you have a dealer do it!!! 1/2 that if you do it... the job is not hard, just time consuming.
Burning Brakes
Joined: Feb 1999
Posts: 1,157
Likes: 3
From: Encino CA
Re: LT1 stumbles under high load (rmd8136)
Could be opti and/or wires and/or plugs. I have a 95 and had similar symptoms--hesitating under load and also stumbling at idle when AC switched on. Dealer replaced opti and symptoms were initially gone but then came back almost worse than before. Brought the car back and replaced opti again but the mechanic also noted that the opti failure (first time?) had caused faulting of the spark plugs and wires, so those were replaced also (all this was essentially no-cost due to the 7-year 70,000 mile emissions warranty that some states have). Since that time (June 2002) car has been fine.
By the way, as poorly as the car was running during these times, no codes were ever registered by the onboard computer.
By the way, as poorly as the car was running during these times, no codes were ever registered by the onboard computer.
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Re: LT1 stumbles under high load (bstirn)
Yes, like stated above, check the coil wire, it has a tendency to rub against the water pump, that happened to me with my '95.
The '95 has a vented opti, so moisture shouldn't be a factor unless it got soaked.
My '93, the symptoms were just as you described, I went ahead and left the old opti in, but bypassed the high voltage part and converted to the LS1 coil setup.
Cost is a little more than a standard opti replacement(opti=$400+$300 labor).
But I'll never have to worry about it again.
See my post here:
http://forums.corvetteforum.com/zerothread?id=629638
The '95 has a vented opti, so moisture shouldn't be a factor unless it got soaked.
My '93, the symptoms were just as you described, I went ahead and left the old opti in, but bypassed the high voltage part and converted to the LS1 coil setup.
Cost is a little more than a standard opti replacement(opti=$400+$300 labor).
But I'll never have to worry about it again.
See my post here:
http://forums.corvetteforum.com/zerothread?id=629638
Melting Slicks
Joined: Apr 2003
Posts: 2,150
Likes: 6
From: NS
Re: LT1 stumbles under high load (jfb)
Plugs not firing. Check the resistance of the plug wires, they should measure about 5000 ohms per foot. If you have plugs with a lot of miles on them, install new ones.
My '95 had the same problem. People here said the usual...Opti. Turned out to be spark plugs. Follow the diagnostic procedure in the Service Manual. Check your plugs first, then wires, coil, etc.
Le Mans Master
Joined: May 2003
Posts: 5,362
Likes: 3
From: Delray Beach FL
Re: LT1 stumbles under high load (Atok)
I recently was having the same type of problem. I know the optispark is good, as it is only around 25,000 miles old, if that. I do know that the plugs and wires need to be replaced and have them on order. My hesitation ended up being the throttle position sensor. Under a full load, the TPS was not acting right. I replaced the sensor and the stumbling went away. It was only happening when my Vette was around 215 degrees or hotter. At cooler temps, it didn't stumble at all. May want to take a look at your TPS. It is a heck of a lot cheaper! And could be the source of your problem as well.
:seeya
:seeya
Burning Brakes
Joined: Feb 1999
Posts: 1,157
Likes: 3
From: Encino CA
Re: LT1 stumbles under high load (Whitvette)
One significant clue that helped the mechanic to identify my problem was that I could get the "miss" to occur at idle by just switching on the AC. It seems that the added load on the engine was enough to cause the faulty ignition components to break down.
