I have looked at lots of posts that say diagnose the problem don't throw parts at it. How do I diagnose this? The car gets warm and after about 20 min of driving it wants to cut out or as I call it "buck" when I ask it to perform. Only does it when it is warm, doesn't have any codes, if I drive it normal hardly ever have a problem, doesn't do it at idle, only under load usually in 3,4,5 gears when I mash the pedal, and always in 6 climbing a hill. Where do I start so I don't throw parts at it? And no I don't have a FSM yet. Car is a '92 LT1 6 spd. Any help would be GREATLY appreciated.

 

First step is to see if you're getting any diagnostic trouble codes (DTC's). Search the forum for info on how to display codes (all you need is a bent paper clip).

Second step, if you've access to a laptop, is to get a free copy of TTS Datamaster and the OBD (on-board diagnostics) connector for your year Vette. The software will track sensor readings, DTC's, etc for (I think) 30 minutes per recording session. You get 20 free sessions before you have to cough up a $100 license fee, after that it's unlimited recording.

Dunno about after that.... I'm still in the recording/analyzing stage with a hot engine stumble on my 94 LT1.

Good luck!

 

Probably not what you want to hear but the FSM is still the starting point. In addition to the step by step troubleshooting fault trees or charts for specific codes there is an even less used section called Symptoms that addresses concerns such as you describe when no fault codes are present.
A scan tool or PC cable and software can be useful as fuzzydawg mentioned to view live sensor data, reading codes is only one function of a decent scan tool. Other tools that will be very helpful are a digital volt ohm meter and fuel pressure gauge.
Resist the urge to buy the very cheapest ones, just as the genuine factory service manual is worth much more than the Haynes comic book. For those who are not familiar with using a DVOM, purchasing one that has autorange capability can be very helpful since you do not need to choose the correct range to get an accurate reading and they only cost slightly more than the most basic tools. Fuel pressure gauges are no different, the cheapest ones invariably have a very short hose, spend more to get one with a hose long enough that you can tape it to the windshield while driving so you can observe what happens under load.
A decent inexpensive substitute for the FSM is an annual subscription to ALLDATA, for less that $30 you can have access to most of the same information and some that is not included in the FSM. Another benefit is that with a credit card you can sign up and have virtually instant access instead of waiting for a manual to be delivered.
One last suggestion is to simply do a very thorough visual inspection with emphasis on vacuum hoses and secondary ignition wires, do that part at night to spot arcing wires that can cause issues such as you describe. Good luck.

 

Two EXCELLENT recommendations.


If $$$ are an obstacle, your can visit shbox.com and GrumpysPerformance.com for their trouble-shooting posts on many of our most common issues.

Jake

 

Yep, they are correct, you can't beat the FSM. Old wrenchers will tell you that a miss/stumble under load is most likely ignition connected. That is where I would start while I waited on my FSM to arrive.

 

Thanks for the excellent advice! I have a laptop where do I get an OBD connector for a '92? I also have a DVM don't know if it does ohms but I do have to turn a **** to tell it what I want it to do and I have no idea how to use this tool. I need to find a fuel pressure gauge will autozone or orileys have good ones? I will check out the online resources too. I have checked under the hood at night and didn't see any arcing. I also suspected the coil but the parts store said it couldn't check it hot, had to be off the car. Thanks for the help.

 

On a DVM, the ohm symbol usually looks like a horse shoe with feet. It should read infinite when the two leads are apart and Zero when you touch them together.

 

Good info.
Haynes comic book- Thats funny

 

Thanks on the ohms I have that. What do I need to set it to the check other stuff. I'm pretty dense when it comes to using this tool so any detailed help would be great.

 

The Datamaster software is at this site:

http://www.ttspowersystems.com/DataM...ownloads2.html

For your 92, you need DM35xDA.


Cables (including OBD-I) are here:

http://www.aldlcable.com/



Good luck!

 

Try these links for a pressure gauge and DVOM, but don't get hung up on buying tools because KNOWLEDGE is what it takes to fix cars, the tools are just a means to collect it.

http://www.amazon.com/Actron-CP7838-.../dp/B0009XQUKC

http://www.amazon.com/Equus-3320-Aut.../dp/B000EVYGZA

Once again I must say the FSM has a very helpful tutorial at the beginning of the electrical section that does a decent job of describing basic operation of volt/ohmmeters and other electrical test tools.

 

FIRST test with ANY efi engine 'driveability problem' is FUEL PRESSURE...that's why the factory puts the test port (schraeder valve) so ez to access on almost all models...tester for under $20 from harbor fright will work.

-my guess(es):
your symptoms sound like 'lean backfire'...today's 'blended' gas/alcohol mix does not match the original ecm air/fuel/timing tables, misfires in some cars (if possible, find a vendor that is not selling corrupted gas and see if problem vanishes)...spark plugs of wrong type/excess gap/multi-electrode/etc may tend to misfire under load , old/bad plug wires same (what's in your walker ?)...a CTS that is 'off calibration' (many are) will cause the ecm to skew air/fuel ratio (unplug elec connector and check CTS ohms at 'engine cold' and also at operating temp with a DVOM or scanner, compare to SM values)

 

When is the last time it saw a real tune up? Plugs wires cap rotor fuel filter PCV valve etc. ? If you are feeling real ambitious relash the valves though seldon needed. If they are 2 years old just change them before making yourself crazy would be my though. Most problems are auto shop 101 and that starts with regular maintainence..

 

Ok took the Vette to pick up my son from school and noticed it wants to cut out at half petal and it has a spark knock, could this still be ignition or do you think it could be lack of fuel? I know I need some tools to figure it out but just wondering what to buy first, fuel pressure or OBD cord.

 

Ha Ha! I trouble shot how to get an avatar and I was successful!

 

I know this is going to be stupid. Just going by what you said in your first post esp in 6th pulling a hill. Could you be lugging the motor to much. In other words instaed of 6th pulling the hill did you see what would happen in 5th or 4th. that will give you the bucking and spark knock. just asking is all

 

LOL no. I know how to drive a manual transmission. I've had one since I was 15. But thanks for checking.

 

I have only had the car for a short period of time and I have not performed a Full Tune Up. I have changed the fuel filter, replaced some vacuum hoses, and put some gas treatment in the tank. I didn't understand what I need to do because it is 2 years old, replace it all plugs, wires, cap and rotor? I hope you aren't suggesting I replace the cap and rotor every 2 years, that would get upsetting as well as expensive.

 

With no codes, the ECM thinks everything is ok and it's neither extremely lean or rich (because there would be a code if it was) so I'd guess that some device is sending garbage or that something that isn't monitored is acting up. It's also a hot/load problem and if that load is WOT the 02's are out of the picture and there's little that a scan will show you.

Fuel delivery is a function of the Coolant Temp Signal so that's the one piece of data I'd want to see with my scanner - particularly since this is a "hot" problem. Starting with a cold engine, it should read ambient and then rise to the thermostat; drop a degree or two when it opens and then rise to the threshold for coolant fan operation. Otherwise, the more logical piece would be a fuel injector or injectors so check resistance across the injector coils cold and hot. They should all be 15 to 17 ohms.

If they're ok, this is an early Opti so there's a high probability that yours is toast and depending on mileage, maybe never replaced - check recalls against your VIN.

If you still can't find it, you can try isolating it to a cylinder, but with an opti it's tough without a tool to kill each cylinder - pulling a plug usually does nothing as the ECM will compensate and since this is a load problem doing it in your driveway probably won't tell you anything. As an alternative, check compression or if high miles, measure valve lift; look for a broken valve spring.

 

The mention of spark knock would make me look at fueling first assuming of course, that you have good fuel in the tank. A pressure gauge as mentioned and checking the cts is certainly a good place to start. I would want to know were the f.pressure is set at, and that it is not dropping under load (I would also check the regulator).
I like using a scanner and or a laptop with software, as it is easier and faster to see all the inputs from sensors etc. I'm not convinced you need to change the wires etc., however checking them for cracks, burnt etc., and pulling the plugs (seeing how they look) would also be routine.
As to buying which product first...ultimately with these cars you will probably end up with both.