stumbles but go,s

[timbo]

Ive posted the problem before took advice and changed out plugs with ngks.............first 91 gl injected...starts great idles smooth startes out fine but going thru acceleration its like somone is tapping a kill switch all on or all off...........i can get down the road and home again but only by play ing with the throttle...have tryed playing with tps adjustments no noticable change? help

[Kraig]

TPS is bad?

[JohnGunn]

I'm trying to get a better idea of how the problem would look if I were there.

You say it starts and idles well. So if you shift into 1st and start to accelerate through the gears when would you first notice a problem, in what gear; at what rpm?

It sounds like the problem shows itself first in acceleration, and the fact that you have to work with the throttle to get home also tells me that the engine will also not maintain a constant cruising speed. Is that correct?

When you are struggling with the "on and off" jerking what happens if you suddenly push the throttle to the floor? Will the jerking stop as the car accelerates?

Is there a difference in how it reacts while at a cruising speed depending on whether you approach that cruising speed from a speed above or below?

Finally, do this test for me. Start with the engine warm and idling. What is the idle speed? Then using the accelerator cause the speed to rise by 200 rpm, e.g. from 700 to 900. Hold that speed for about 5 seconds, then go up another 200 rpm and hold, etc., up to 3000 rpm. Is there any missing in this test? If so at what rpm does it start and at what speed does it perform the worst?

John Gunn
Coronado, CA

[1988redfordfestiva]

i would say check wires but if this the efi i think the disty can be at fault

[mechanicaldj]

wet distributor cap

[timbo]

john just got your answers......its in all gear it cuises with noticeable stubble (mild) stand on it may bog then take off or take off and then bog on and off tps sensor? how would i test? timbo

[1988redfordfestiva]

i have a link to show how to test the tps

http://www.youtube.com/watch?v=vyxNLrCMk3I&feature=PlayList&p=CF23387B5766E81E&playnext_from=PL&playnext=1&index=43

[JohnGunn]

john just got your answers......its in all gear it cuises with noticeable stubble (mild) stand on it may bog then take off or take off and then bog on and off tps sensor? how would i test? timbo

When faced with a driveability problem my inclination is to eliminate ignition problems before moving on to the fuel system and the emission control system which controls fuel delivery.

You've changed the spark plugs and I'm assuming that made no discernible difference. But there are other parts of the ignition that could be failing, first that comes to mind is the ignition module.

You didn't do the test I requested in the last paragraph of my last post. I would still like to know how the engine performs at various rpm with no load and the transmission in neutral. If the problem demonstrated itself in that environment, you might spray the ignition module with something like brake kleen or carb cleaner, etc. and see if that affected the problem. The on/off behavior sounds like what you might expect from an electronic component which was just at the point of failure. Do that test and let me know.

The general wisdom seems to be that this kind of problem is caused by the throttle position sensor. It is pretty easy to speculate that it might be sending inconsistent data to the computer and causing the erratic behavior. It is true that the computer uses the throttle position sensor to tell it when the throttle is closed so that it can shift into deceleration mode. And in that mode it will quickly shut down the flow of fuel to the injectors. Sounds perfectly reasonable. But the truth of the matter is that the computer is really a "black box" that is not well document in the Service Manual, and perhaps is even protected as a company proprietary secret. I honestly believe if you have a rich imagination you should be able to make a convincing case for any emmision control device or sensor to be the cause of any driveability problem.

If I came to the point of suspecting the TPS I might disconnect it to see how the character of the failure changed. That would also go for the Vane Air Flow Meter as well, another part whose erratic behavior could conceivably cause sudden on/off transitions.

I think I would check for vacuum leaks with starting fluid, and clean connectors to coil, ignition module, distributor, TPS, O2 sensor, and VAFM using spray cleaner. Protect any electrical tape from that spray.

I wish I could be more definite. Maybe after I've slept on it, I'll have other ideas.

John Gunn
Coronado, CA

[timbo]

Thanks john ill run that test and get back you.....

[Aaronbrook37]

Check the tightness on your belts too... a loose/dead alternator belt can do this to a car.