ttt

 

I have a question...is it possible that an injector is going bad? Would a bad injector (or two) cause a low rpm, miss under load?

Drew

 

Add another one to the list - new plugs (replaced originals with 95k - discs were gone, but otherwise textbook coloring), TPS, coil, ICM, PCV, fuel filter, disco'd EGR, cleaned MAF, rescreened MAF, unplugged MAF, unplugged MAPS - zero improvement to the problem.

Car starts, idles and revs up PERFECT after sitting all night. Once it is warmed up driving around, there is a stumble that will occur when the engine rpms are between 1250 - 3000. Above that rpm, the engine does not miss (or is not noticeable).

The real problem will occur, for example, when driving the car for 30 minutes and letting the car sit for another 30 minutes. After that, it will start hard, misfire, stumble, backfire in the intake and sputter. While trying to start the car, constantly feathering the throttle will keep it running for a minute or two and then it will die. Let it sit for another 30 minutes to cool off and it'll start and idle perfectly again. This will set a P0300 code - multiple cylinder misfire (no other codes).

Obviously something is getting heat soaked and is causing the problem. I've read lots of posts about "replace the Opti$park & wires" and others who have done that and had no improvement. This all leaves me hesitant to plop down the cash on that along with new wires and I suppose do a water pump as long as I'm there. Then if that doesn't work, new 02 sensors, and then god only knows what else I could spend money on. ARGH!!!

Special ordered my Z brand new in 1996, had no major problems until about the last 18 months and now it is nickel and diming me to death - more like $100 and $1000 me to death. ACK.

For Sale - Adult owned, One owner, Never raced, Non-molested, '96 Z28, 6 Spd, 95k interstate miles, All Options, Black w/ Tan Leather, Tinted Windows, Moser 12 Bolt w/ 3.73s, new SS 10 Spoke 17" wheels with GY F1s, Slight Hesitation Problem

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The more I look at the engine bay of my new Silverado SS, the more I want a F-body or C5 with a LS1. I can see all the spark plugs!

 

man if your car wasn't hesitating like mine, I would be all over that!! Buy!!!

let me know if you get the hesitation fixed!!!

Thats the thing that bothers me the most, my car runs SOOOOOOOO nice, when it's cold, I LOVE IT, thats why I bought it!! cause it did run SWEET!! and then when it gets heat soaked, it runs like a 1975 Camaro or something
I really need to get this fixed,
It's sad really, I know it's got to se something stupid!!!!!
If I new RIGHT now that it would cost $500 to get my car fixed tomorrow, and I was guarenteed it would BE fixed, I WOULD FREAKING pay that so fast!!!

IM SICK OF HESITATION!!!!

 

Well i have a 1993 Z-28 ... I have replaced the OPTI , WATERPUMP, plugs, wires, map, o2, coil , PCM , ECU,fuel filter .... And I still can't fix the spudder.. It pops out of the intake and hestates real bad when it gets hot "only at low rpm". Some times it does it when its cold. I am going to clean that stud and if it fixes the prob cool. But i seriously think i got a bad opti installed or the IAC is clogged. Any help you guys can give would be great!
Sinister SS you have a bad opti!

 

Mine ran like crap when warm, too. Put in a 160* thermostat and used a Hypertech Power Programmer to change the fan on/off temp setting and it runs cool and strong all the time. Then I got the low rpm stumble under load -- a year after the water pump died and bled all over the opti. The opti was replaced and it runs sweet now.

Putting in a low temp stat w/out changing the fan on/off settings will still let the car run hot in traffic or idling. The fans don't come on untill it hits 210-220*. My fans kick on at about 180-190* and it runs strong even in stop and go traffic.

A cap and rotor kit just replaces the high voltage parts in the opti but the particles erroded off the old cap and rotor are still floating around in the optical part of the opti and thats what makes them run like crap.

 

For the love of humanity someone please solve this problem!!!!!!!!!!! My car is running this way now!!!.

 

I had the same problem but this is what solved it?? HEADERS, 1.6 roller rockers! I swear after I did these mods I watched my temp go so close to 210 and then when I give it gas it does not hesitate!! WTF!!

 

have any of you guys with a problem similar to mine tried disconnecting your battery and resetting your computer?like I said,when I disconnect mine for a while,(like overnight)and reconnect it,the hesiation and knock goes away for like 5-10 miles of driving even if temp is around 170,when I first started disconnecting it it would go away for 50+ miles Its gotten wise to my tricks,also If I disconnect and reconnect it while the moter is warm,the problem does not go away at all.I hope to change the opti this weekend (Jason cromer is out of stock supposedly till tomorrow,not surprised)and Brian at pcmforless has no cores until monday so it looks like I'll be doing the opti first,but is there anyway if my pcm is bad it can short my new opti or vice versa?

 

A few things to consider...

Modern fuel injected engines run great when cold, compared to older carb'd cars. When they go into closed loop, they can actually run worse, as the mixture typically leans out.

The LT1's torque curve builds very quickly between idle and 2000 rpm, and is relatively flat from 2000 rpm and above. Problems will be very pronounced between idle and 2000 rpm.

Trying to solve a hesitation problem on a car with a 100k mile opti is a waste of time. Replace the opti, plugs and wires first. They do wear out, and will cause plenty of problems on their own, such as missing.

The LT1 uses electric fans, not a belt driven fan. As a result, the engine experiences greater temperature swings, especially at idle.

A leaky intake manifold gasket can suck oil INTO the intake ports. This will be most pronounced at idle, when vaccum is high, and when the engine is very hot. Airflow is low, so the relative amount of oil is high, and can give the O2 sensors a false rich condition. The PCM will lean out the fuel mixture to compensate. As you accelerate , vaccuum drops, airflow increases, and the relative amount of oil being sucked into the manifold decreases dramatically.

My '94 has 176k miles, and I've replace the opti, plugs, wires, coil, ICM, EGR, O2's, waterpump, exhaust gaskets, etc. It still had the hot hesitation problem after all of these items were replaced. I recently replaced the intake gaskets to fix the oil leak, and this alone has made a HUGE improvement in the hot hesitation problem. I was carefull not to change anything else during this repair, and even left the battery connected.

 

Good post!



How many of you have gotten a scanner ,voltmeter , fuel pressure gauge, etc, and had your cars scanned, and if you understand the readings and parameters took the time to check things or have someone who does understand to look at them if you don't. And when I say scanned , I don't just mean look for trouble codes. A car can have a problem , and not give a code or SERVICE ENGINE LIGHT sometimes. Maybe the sensor could be not functionig correctly, and even though it is bad enough to cause the car to run bad, it may not be out of a certain range to where it will throw a code or light. Plus maybe there could be a problem with the computer.

That's where knowing what the readings mean and knowing what you are looking at comes in. Alot of you are going to be throwing parts at your cars for years if you don't do some diagnostic work. For the price 1 or2 of those sensors you could get a good voltmeter . A diagnostic tool cost more, but if you don't have one, or don't plan too get one, maybe you could borrow one or have someone with one scan the car for you.


The 2000 rpm miss is NOT a normal problem with lt1's . VERY COMMON, but not noral. I test drove quite a few cars, before i got mine, and none of them had it. My car does however have the stumble ,miss, but I am going to try and wait and get a voltmeter, and scan tool. I think if more people did more diagnostics and testing, instead of throwing parts at the car, these problems would be fixed for more people.

Also, for those that are thinking the problem is heat soak if your car developes this problem when it warms up, that does not neccesarily mean heat soak. It could be the certain sensors , which do "there thing" after the car reaches a certain temp or is run for a certain time period.

 

so, my question is, do you think it's possible that my fuel pump "could" be going out??? or would my car act COMPLETLY different if that was the problem???

Thanks for all the information, every time I read this web site, I feel like I am getting closer to fixing my problem!!

 

I bought a cable so i can hook my laptop to the car and scan it with datamaster. Maybe that will help solve the problem. Can a leaky manifold gasket really leak oil into the combustion chamber!?

 

ok so just today, my car after running for awhile, the "Low Coolant" light comes on????? So I turned the car off, waited a minute, then it starts to flicker for a few seconds, then it stays on??
the thing is!! My car ALWAYS has the coolant full! I ALWAYS check it, among other things, When it's hot, the Coolant is RIGHT where it should be, and when it's cold, IT STILL is in the right spot.!

 

I know how you guys feel. I'm the king of Low RPM Stumbles. I've had it for over a year now and I've done everything except for rebuilding the engine. You guys might want to search for some posts under my name. There's alot of good info in them.