Recently my car developped a problem that noone seems to know how to solve. The car runs great exept when I am cruising between 45 and 70 mph. At thhose speeds the car has an intermittent miss. It's not a bad miss, just enough to anoy the **** out of me. I have changed the plugs, wires, cap, rotor, pickup coil, checked all vacume lines, adjusted the valves, checked the fuel pressure, and changed the fuel filter. I have taken it to three mechanics, had diagnostics run, scanned, and had an engine analyzer put on it. No one can find a problem. I removed the egr to see if that was the problem, and that didn't help either. Anyone have any ideas?

 

Is your cat clogged?

 

since you say it happens between 45 and 70, i will assume it runs fine at any speed faster or slower. three things come to mind.
1) tps sensor - when you are traveling a general speed there is a certain load on your engine which requres you to put your foot down to roughly the same spot every time you go that speed. check it and make sure the voltage increases steadly, no sudden jumps.
2) MAF or MAP - the only other sensors on the engine that deal with airflow and load.
2) transmission related - particularily the vechicle speed sensor. i believe it is supposed to help determine fuel/spark needs for the load on the engine.
some one should correct me if i am wrong..

 

How can I check the speed sensor and MAF sensor?

 

you can test the maf by having the care idling and taping the maf with a screwdriver. if it stumbles or dies then the maf is bad. there is a voltage check for the burnoff relay but i don't know how to check it. as for the speed sensor, maybe some sort of pulse reader like the kind you check your injector pulses with; to make sure you have consistant pulses. - just hypothisising

luck.
jess

 

The maf and tps are good. As for the cat, it is only 2 years old. It should still be good. I don't know how to check the speed sensor, so if anyone can tell me I would appreciate it. Any other suggestions on where to look for problems would also be appreciated. Thanks.

 

86 TeePee Eye,

The VSS generates square wave pulses for the speedometer and the ECM. failure of this sensor usually results in a DTC 24. If you want to test yours, unplug the VSS behind the instrument cluster and drive the car at 45 MPH. You should generate a MIL lamp and code 24 in short order.

An easier way would be to drive the car with a moderate load and wait for it to shift into torque converter lockup. The ECM will not allow the TCC relay to engage if the speed is not above either 28 MPH or 41 MPH (depending on what EPROM you may have). If the VSS isn't working, the TCC will never engage. Your tachometer should indicate when teh lockup occurs.

 

If you get a code scanner, the type with the digital readout, it'll tell you the tps voltage in real time. Either go to a mechanic for this or some parts stores will loan one with a fully refundable deposit. The store I used to work at (Murray's Discount Auto) loaned the scantool for 200 and the GM cartridge for 100 but you got all your money back so it was a good deal and it's a great tool.