Hi all. I have a 94 pontiac formula 5.7LT1 with 210,000 miles. It has developed a part throttle stumble with occasional backfire.WOT car will back fire and then pull hard to redline smoking the tires all the way even with knock retard pulling 8 or 9 degrees of timing. Data logging shows all sensors appear to be functioning properly including 02 sensors. ECU is callling for knock retard at all throttle settings even when knock count is not incrementing. No SES codes have been set. Opti is only 5 months old and I just replaced plugs, coil, coil wire, ICM to no avail. LTFT and STFT appear normal. I am thinking about changing out the original 02 sensors and fuel pump Just because they are due being 12yrs and 210k old, but hate just throwing parts at a problem. Could my timing belt have jumped a tooth ?? Any Ideas ????
Help with part throttle hesitation
- Thread starter 454MAG
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Thanks for the replies. I have checked for leaks and recently replaced the EGR valve which was throwing a code. As far as dealer service been there done that. They are notorious for throwing parts at a problem until they get it plus charge you 60 bucks an hour and list price for parts---NO thanks. I dont mind throwing parts at it because I have not made a payment on this car in 6 years. I have datamaster software which allows me to read/store all sensor data in real time but eveything looks good and no codes are being set.
thanks Ted
thanks Ted
454MAG,
I know this isn't specific to your vehicle, but with having spent time working with carbs, I feel the engine stumbling upon throttle would be due to the fuel enrichment system.
As one throttles, the sudden drop in manifold vacuum would cause fuel to drop out of suspension by accumilating into larger droplets. These droplets would not only mix less readily with the incoming air charge, but would also collect on the runner and cylinder walls, thus leading to a sudden leaning of the fuel-air ratio. The results of which would be engine bogging w/o exhaust smoke, hesitation, stumbling, or if bad enough, stall.
To solve this, a manual fuel pump was added. It was a spring-interfaced design that was actuated by linkage off the butterfly shaft of the carb, and would sqirt a metered amound of excess fuel into the intake system. This enrichment device was called the accelerator pump.
With the change-over from carb fuel-air management to electronic fuel injection, I believe the cirumstances of conditions still exist since fuel is mixed with the incoming air charge behind a butterfly valve system. The controls and monitoring for such is much different however, and to which I am not confidently versed as yet.
Good luck, and hopefully someone with great experience will report soon.
I know this isn't specific to your vehicle, but with having spent time working with carbs, I feel the engine stumbling upon throttle would be due to the fuel enrichment system.
As one throttles, the sudden drop in manifold vacuum would cause fuel to drop out of suspension by accumilating into larger droplets. These droplets would not only mix less readily with the incoming air charge, but would also collect on the runner and cylinder walls, thus leading to a sudden leaning of the fuel-air ratio. The results of which would be engine bogging w/o exhaust smoke, hesitation, stumbling, or if bad enough, stall.
To solve this, a manual fuel pump was added. It was a spring-interfaced design that was actuated by linkage off the butterfly shaft of the carb, and would sqirt a metered amound of excess fuel into the intake system. This enrichment device was called the accelerator pump.
With the change-over from carb fuel-air management to electronic fuel injection, I believe the cirumstances of conditions still exist since fuel is mixed with the incoming air charge behind a butterfly valve system. The controls and monitoring for such is much different however, and to which I am not confidently versed as yet.
Good luck, and hopefully someone with great experience will report soon.
Check the obvious first, look for any vacuum leaks. Then check the egr valve and check out the pcv valve also. Its a good bet to change out the O2 sensors with that many miles anyways. I would also take a look at the fuel pump pressure and fuel injectors.
I also want to add sometimes you are ahead if you take the vehicle to the dealer or a good mechanic. If they find out something you cant, you might save money over putting lots of parts on it yourself.
If you do the EGR, replace the solenoid as well. Cheap insurance.
I'd also clean the MAF. Take it off the car, open it up (i.e. remove the screen) and use electrical contact cleaner or something similar (alcohol, carb cleaner, whatever) and CAREFULLY q-tip the wires until the black crud is gone.
I'd also clean the MAF. Take it off the car, open it up (i.e. remove the screen) and use electrical contact cleaner or something similar (alcohol, carb cleaner, whatever) and CAREFULLY q-tip the wires until the black crud is gone.
454, a worn TPS is a possibility. Take a voltage reading on the signal wire, and see if it advances from about 1v to 4v smoothly, when the throttle is advanced. If you see a spike or momentary open, you've found the problem.
You've probably got a 3-wire TPS, with one wire being a 5v reference, one ground wire, and one signal wire. You can push a straight pin into the connectors and take the voltage readings off the pin, preventing damage to the connectors.
You've probably got a 3-wire TPS, with one wire being a 5v reference, one ground wire, and one signal wire. You can push a straight pin into the connectors and take the voltage readings off the pin, preventing damage to the connectors.
Kevm and loneranger thanks for the replies. I actually cleaned the MAF last week thinking maaybe it had oil from the K+N on it but that made no difference. I have compared the output of the MAF and IAC and all other sensors to stored datamaster files from a year ago and all seem to be functioning normally. I just bit the bullet and bought another new opti. The one I installed in 9-04 has a 1 year warranty from GM and the parts guy said some do go bad that quickly. The last two optis threw a code for the optical section maybe the cap and rotor went bad this time. It amazes me the car can run so bad and all sensor data looks good and no codes are being set. If this opti was not under warranty I would install the Delteq ignition setup using northstar coil packs. Maybe the problem was caused by running Mobil 1 0w40 instead of the tried and true Mobil 1 10w30/15-50 mix.JK
Ted
Ted
My Camaro also has a drivability problem with everything in datamaster looking normal. Bugs the crap out of me. How is your mileage? Mine's pretty bad, and that car has no prayer of ever hitting the EPA highway estimation of 26mpg.
Mine is a sort of stutter/light chuggle/surge from idle to about 2000rpm, then it's fine. According to a popular camaro forum, it is an INCREDIBLY common problem, even after replacing practically EVERYTHING. The worst is, the problems aren't identical, as different fixes work for different people. Like "finally replaced my opti and it's fixed!." Or "just did plugs and wires and she smokes the rears again!" Or "On a whim, I ordered some SVO injectors and the hesitation is gone!!!"
Mine is a sort of stutter/light chuggle/surge from idle to about 2000rpm, then it's fine. According to a popular camaro forum, it is an INCREDIBLY common problem, even after replacing practically EVERYTHING. The worst is, the problems aren't identical, as different fixes work for different people. Like "finally replaced my opti and it's fixed!." Or "just did plugs and wires and she smokes the rears again!" Or "On a whim, I ordered some SVO injectors and the hesitation is gone!!!"
not familiar with this particular car, but it sounds like not enough fuel is being pumped in. Could be clogged injectors.
Most likely it's the MAP sensor. Remove it from the intake and shoot some carb cleaner into the hole of the sensor. This trick has fixed alot of the problems that you talk about.
Thanks again for the replies. Kevm, The fuel mileage is still excellent even with the miss I am getting 23.8 to 24.6 MPG on a mostly hwy ride 100 mile commute. this car has always ran excellent with no issues like this. I will have a new OPTI in it tonight and expect that to fix all problems. Unfortunately the one I just had flown in was smashed to bits because the idiot in shipping just threw the opti in the water pump box with no packing.
I will clean the MAP sensor and did consider replacing it but all MAP sensor values look very similar to datamaster logs from a year ago. I can only take 8 samples per second so maybe I could miss an intermittent spike.
So far 3 times iin the past 10 years this car has given me a problem. Twice it was the opti and once it was the coil wire.
I also considered a fuel problem but If it was a low fuel pressure problem I dont beleive it would pull so hard to redline. I can still smoke the 245-50 tires all the way thru first gear. I ordered a fuel pump and will probably install it the next couple of weeks. sooner if the opti does not fix things. With the age and miles on the vehicle I dont mind replacing things before they fail. It still has the original staarter and alternator beleive it or not.
Ted
I will clean the MAP sensor and did consider replacing it but all MAP sensor values look very similar to datamaster logs from a year ago. I can only take 8 samples per second so maybe I could miss an intermittent spike.
So far 3 times iin the past 10 years this car has given me a problem. Twice it was the opti and once it was the coil wire.
I also considered a fuel problem but If it was a low fuel pressure problem I dont beleive it would pull so hard to redline. I can still smoke the 245-50 tires all the way thru first gear. I ordered a fuel pump and will probably install it the next couple of weeks. sooner if the opti does not fix things. With the age and miles on the vehicle I dont mind replacing things before they fail. It still has the original staarter and alternator beleive it or not.
Ted
Just to update. Installed new OPTI and all is well. Ran some data logs and G-tech times and car is pulling a 13.7 QTR mile at 101 with a 2.0 60 foot with 0 knock retard. NO traction lots of tire smoke and big smiles on our faces. Not too shabby for 210,000 mile GM product with original motor and transmission.
BTW it was not stone auto that shipped parts. The parts guy actually shipped me a new one ups red at his expense and covered the old one under warranty.
Ted
BTW it was not stone auto that shipped parts. The parts guy actually shipped me a new one ups red at his expense and covered the old one under warranty.
Ted
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