Feels like Rev Limiter at 4,500 RPM

[IceyVert]

Our for a drive yesterday and noticed a slight hiccup/hesitation under WOT at 4K and again at 5k rpm. Got home and noticed a loose plug wire. Snapped it back in and figured good to go. Out for a drive today and now it’s much worse. Once it gets up to 4,500rpm it acts like it is bouncing off a rev limiter and will not continue to rev. Does it under both partial throttle and WOT. Under no load it will do the same at 5k rpm. I have a new fuel filter and plug wires waiting to go in. Plugs and air filter done 1,000 miles ago and I checked for vacuum leaks with propane and found none. Anyone have any ideas what to troubleshoot? Car drives fine with normal power up until 4.5k.

[ADEN]

Tps.

[moatilliatta]

Or loose crank wheel.

[IceyVert]

Crank wheel? Do you mean CPS? Sorry if I’m not familiar.

[roguetoaster]

Crank wheel? Do you mean CPS? Sorry if I’m not familiar.

Crank trigger wheel is what he means. The "pulley" on the front of the crank with a toothed wheel around it that the CPS references for crankshaft position.

[Gregs///M]

Failing AFM.

[IceyVert]

I should add occasionally I’ll have an extended cranking time (typically when hot). Does this point towards crank position sensor??

[IceyVert]

Cap, Rotor and Wires changed. Now does it at 4,100rpm. Definitely runs smoother and starts better, but same issue. Does it at part throttle, full throttle and in N/P. Stomp test revealed no codes - 1444. Guess crank sensor is next.

[Gregs///M]

Typically when the CPS fails, the car will never start.

[e30davie]

Based on this thread you should replace everything :)

If it were me i would be getting the bently manual out and systematically testing all of the above mentioned sensors, as well as the coolant sensor and any other sensors not mentioned. There are testing regimes for all of them. And if they all check out then start looking at fuel supply.

[IceyVert]

Finished bench testing the AFM and I don’t think it’s causing the issue. Definitely will test what I can. Fuel filter and coil will be changed Friday, fingers crossed.

Question, if stomp test works does that mean TPS is good?

[e30davie]

Maybe, but test it anyway. it only has two switches in it. 100% and 0%. its not a variable TPS like other models.

You also need to make sure that the TPS is adjusted correctly so that when you go wide open and when it comes back to closed it engages the switches properly.

[Gregs///M]

Finished bench testing the AFM and I don’t think it’s causing the issue. Definitely will test what I can. Fuel filter and coil will be changed Friday, fingers crossed.

Question, if stomp test works does that mean TPS is good?

I saw you tested your AFM with a 9V battery and meter, those are both the incorrect tools to use and will cause you to have false positive test. You must use a 5V supply and a oscilloscope. 9v battery is almost double the voltage the AFM normally sees so if you have a voltage drop, that drop may be there with the DME 5v supply but not there with a 9V battery. A meter displays an average of the voltage, voltage drop can occur in a microsecond and you meter will never be close to catching it. Plus you cant test if the AFM is good or bad if you are looking at the average voltage. Only an oscilloscope can catch a microsecond voltage drop. See my AFM testing thread.

Yes, the TPS can still be misadjusted even though the stomp test initiates. Perform the testing procedure for the TPS in the Bentley repair manual.

[LJ851]

It's the crank sensor. I had one fail with exactly the same symptoms.

[IceyVert]

New coil and DME relay, issue persists. Fuel filter Friday and then I guess I’ll throw a crank sensor at it.

Tested AFM while hooked up to 5V and performed as expected. No dead spots and max opening = max voltage, 4.53V