I recently purchased a ’91 N/A 5 speed. The engine starts and idles fine and can be coaxed to smooth high revs. When you begin to open the throttle it stumbles and nearly dies. If while it’s idling, I manually just crack the upper secondary throttle plates, it will rev and stall, rev and stall, almost as if there’s a switch being turned “on and off”. While this ‘on/off “ bucking is going on and I depress the forward most TPS plunger the revs will smooth out. In trying to diagnose the problem, I’ve sprayed carb cleaner around the manifold and vacuum lines looking for leaks, plugged each vacuum line, check the timing, unplugged the pressure sensor and adjusted the TPS with a volt meter. My thoughts are that when I begin to open the throttle plates, I’m changing the manifold vacuum and that’s sending a signal somewhere that’s alternately killing the signal to the fuel injectors? Hoping someone recognizes the symptoms and can help me solve it. My only guess is a bad ECU or TPS? Thanks for your help.

 

A TPS problem is most likely. The FSM has testing procedures for it; FSM links in the 2nd gen FAQ sticky.

You may also want to check for any trouble codes: http://www.johnr.com/cpucodes.html

 

Good idea, but it didn't solve it. I've since replaced the TPS and the ECU without improvement. I opened up the the wiring harnes to look for any tried and cracked wires, but all looks good. The only other cue is that I found that the secondary injectors aren't working. If I nurse the car to high revs and unplug the injector leads, there's no change in the engine speed or tone. If I don't figure it out soon, I'll have to take it in. Ooch. Does anyone have any thoughts. Any help is appreciated.

 

The secondary injectors only fire under load, so just free revving won't activate them.

When you replaced the TPS, did you adjust it properly? Note that the car needs to be fully warm, or the cold start cam needs to be disengaged manually.

Are you SURE there aren't any vacuum leaks?

 

Using a volt meter and probing the data lines I've adusted it ten ways to sunday. There are subtle changes, but I still can't tune out the flat spot. I've ruled out vacuum leaks, as I sprayed everywhere with carb cleaner with no changes in idle speed. I'll spend more time with it this weekend. I sure I've missed something fundamental and it's staring at me. Thanks for the help.

 

And check the fuel system as well such as filter, pressure, wiring etc.

 

Test the boost sensor on the passener shock tower as shown in the FSM.

 

Problem solved! I finally learned how to capture the error codes by watching the flashing check engine light. I did't mention it in my original post, but I had deleted the oil metering pump. This car is being built for an upcoming 24 Hours of Lemons race and we'll run it on two stroke oil. Along with all codes created by my fumbling around, there was a code for the oil metering pump. Turns out that when you remove the pump or it fails, it throws a code and the ECU goes into limp mode. That was the heart of the problem. For testing purposes, I reconnected it and just laid it on the top of the engine. Once all the codes were cleared it ran perfectly. Now that that's solved, does anyone know how to delete the pump and trick the ECU? Thanks for everyones help.

 

You can't delete the OMP from the Engine harness,because the ECU won't see it,and will throw the car into limp mode.
You pretty well got it in hand by keeping it hooked up and hanging it to the side and running premix.That is the best option at this point,..Unless you want to put the OMP back on.