erratic RPM drop and jump at high rpm

[A.Heath_P38]

Hi everyone,

I currently have a issue with my 4.0L (180000km on engine, 416000km on ZF4 Trans) P38 when the RPM exceeds 4700RPM, the RPM's will drop down to about 1300RPM and throw a check engine light when it drops and jump back up to 4700rpm and the light will go off and clear all codes in the ecu at the same time. it jumps up and down around 4 times a second which i feel is quite worrying in the case it stresses the internals with the sudden fluctuations. i only noticed it when i overtook someone who was going painfully slow and it just happened out of nowhere (would like to note that it doesnt happen everytime but often when i need to take it up the rev range) . It Revs all the way to 5500RPM in neutral without a issue and only happens in drive. i dont rev it out much so its not overworked so im not sure why its started happening.

Any answers or suggestions on what to do are appreciated.

If needed I can post a video of the issue occuring for understanding and further context.

Cheers,

Anthony.

[prelude]

Could be all sorts of things but with no error codes my first suspect would be the crank position sensor. it is situated at the left hand side of the engine in the same spot where the starter motor lives on the opposite side. check that plug and clean it out. You can also take the sensor out to see if it has wear marks on it (should not be).

If the ECU does not get a correct CPS signal it won't record an error but also will not inject fuel and send spark, ie you go from full throttle to engine braking.

The gearbox you should be able to test, just shift down a peg or two and rev it the same way and see what happens.

Cheers,
-P

[A.Heath_P38]

i currently have a fault code relating to the crankshaft position sensor, only dissappears when the hiccup happens and comes back after turning the car off and on. so it may very well be that or the flex plate, had my mechanic give me a suggestion, which suggested the flex plate is going out or almost done. car still hiccups in 1st, 2nd and 3rd (dont ask [wink11]). i'll have a look at the sensor when i get the chance.

Cheers,

Anthony

[PeterH]

CPS tends to be a show stopper when it fails, the engine will just die.
You can turn it over, but no spark happens.
When the engine cools down they can start working again.

Yours could be on the way out, that would be my first place to check as prelude suggested.
If you've never replaced yours, it might a good idea to replace the CPS as preventative maintenance.
At the very least, you can rule that out if the problem persists.

[prelude]

The flex plate can be a bit of a thing as well yes. They tend to crack over time.The inspection plate on the bottom of the bell housing should provide you a bit of a look inside, perhaps you need not take the entire gearbox off.

There are a few ways the CPS can give way; age of course but also physical wear. This is usually caused by the flywheel (specifically the teeth of the reluctor ring) hitting the CPS. When that happens there is no proper signal to the ECU and thus the engine cuts out.

In my case it turned out that the plate into which the CPS is mounted had broken loose so under heavy engine load the thing would hit the reluctor ring and cut the engine, when I was lucky it would pick back up again when the RPM dropped but eventually indeed the engine just died (with no errors).

Cheers,
-P