I was driving my 90 vert to Lowes to buy a little SS screw. It was my third trip to the store for various things that day. The car runs fine and has for years. I rounded a corner after sitting at a stop light. I hit 7k in first then just after I shifted to second, the engine just *died*. I darn near hit my head on the steering wheel. Hit the clutch and coasted. The engine was idling perfectly. I put it in 3rd gear and the engine fell on it's face again. Basically, the engine ran fine up to 1200 rpm, then BAM! nothing. It would pulse, sounding like a fuel cut.

I checked under the hood, couldn't think of anything except maybe the MAF plug. I checked it and it was tight.

No check engine light. HMmm. I parked and bought my screws. I drove home on the back roads keeping it under 1200rpm. Then, magically the engine started working right. I drove for a while, through a stop sign. After the stop, the problem was back. Then it was gone, then it was back.

Once I got home I checked out the trouble codes. No codes. I have had the OMP take out the ECU before, but this wasn't really like limp mode at all. Other than the OMP trouble, this car has been flawless for 30-40K miles. It runs like a new car, and still idles perfectly at 800 rmp.

Any ideas of where to start?

thx

Jack

 

How does the car behave free reving while in idle. Does it act fine, but your problem only occurs while under load?

 

When the trouble is occurring, the limit is 1200 rpm, regardless of load, in gear, out of gear.

Out of gear/no load: part or full throttle, when rpm hits 1200 it hits a wall, then speed drops to about 800, then back up to 1200. Kind of bouncy but nothing like a tps idle bounce. If I hold the throttle down and drive, the engine sounds like it is in fuel cut at 1200 rpm till the engine slows enough to catch again.

Then if the trouble goes away, everything is completely normal.

 

Have you tried unplugging the TPS. Perhaps the Fuel Pump Relay and Resistor is acting up. It can easily be bypassed.

 

hook a timing light up to it while you are driving or free reving. Then you can Count out Spark drop off. Then Check fuel pressure and fuel pump and filter make sure its running correctly so you have pressure. If You have spark and the spark is not dropping off and you have fuel pressure then its Signal to the Injectors. MAF has a lot to do with this.
If you have a spare maf try hooking another one up as they do go bad inside from time to time. Sure its rare but it happens

 

Potentially clogged fuel sock or filter?
I only wonder because you say it started after a turn.

 

I was able to pull codes this time:

code 8 AFM
code 10 intake air-afm
code 12 tps full range.


Interesting is that the narrow range plunger is non-functional. It doesn't move. the full range does move. Haven't measured the resistance yet. Still, the car idles smooth and even at 750 rpm. Getting ready to test the TPS and the AFM per the FSM.


BTW, what is the narrow range tps function vs the full range tps function? Just curious if there might be other behavior to look out for.

-Jack

 

AFM tests in spec.

tps coming next.

 

The narrow range tps is dead. Measures .6k ohm whatever the throttle setting. The plunger is not moving in an out, it just sits there. Moving the plunger by hand makes no difference in the resistance reading.

 

Well that's strange. I didn't know the narrow range TPS would stop the engine from revving!

 

I'm not sure it is the only problem. Going to replace it with a borrowed known good replacement and we will see.

 

Narrow range is a full shut, full open read out correct? Maybe since its jammed in the full shut (idle) position the ECU can't detect proper engine speed. There's only one true limp mode and that's OMP. That's like a 3000RPM limit. Sounds like the injectors are not firing enough due to lack of signal from the ECU.

Take the TPS off and try spraying some electrical parts cleaner inside the plunger while opening and closing it.

 

I replaced the TPS with a good one, adjusted the resistance to 1k ohm. Problem solved, car runs perfectly.

 

figured that would be the problem with your .6k ohm reading. the return spring probably died and is grounding out internally somehow.

severe cutouts are generally and often the TPS because the TPS also acts as a fuel cut device.

 

NOW you tell me!

Have you tried taking the TPS from an S4, remove the body from the s4 bracket by bending the tabs and put that narrow range TPS back onto the S5 tps bracket?

 

Interesting idea....does it work?

I've taken S5 N/A TPS and mounted them on a turbo bracket to use. never thought to use narrow range on a S4.

As a sidenote, Jack when you tested your TPS do they have a dead range? I've tested mine in the past (too long ago to remember the actual resistance) and found a dead area in all of them from just around their top end for resistance and then they pick back up on a higher resistance. Again I don't remember the actual numbers I saw. I have the engine in pieces waiting to be assembled. But sometime soon I'll take the tps and get some resistance readings (albiet) they will be cold, not warmed up like FSM says for them to be.

 

Yes, it had a dead range. The entire range. The plunger spring was dead so the plunger did not extend and retract with the throttle movement. Also, when moving the plunger manually, the resistance didn't change. I'd say that had a dead range!

I was set to swap over an S4 box to the S5 bracket then I found a cheap throttle body with tps at a local salvage. I was curious if it had been done.

 

LOL, well I kinda meant if you had tested the replacement.

 

Ah, my bad. No, I didn't test the replacement.