2000 C70 B5204T4 harsh jerking

hss · Post by **hss** » 05 May 2014, 13:10

2000 C70 B5204T4 manual gearbox has harsh jerking while driving with light throttle under 2000 rpm. It happens maybe 3 or 4 times a second and continues as long as I drive like that. That means it has to be driven one gear lower than it should.

It has also a problem that when warm, if you have it idling and very lightly touch the accelerator pedal, the revs drop below idle (to around 500 rpm) and then bounce up as the ECM tries to prevent stalling. Idle is fine if I don't touch the accelerator.

First I thought it is an ETM problem but Vida sweep test passed. I still took it out and installed a Sacer contactless TPS but nothing changed. I have Vida+Dice but the only thing I have noticed is that when applying light throttle at idle, ignition timing jumps from around 10 degrees BTDC to 30 degrees and immediately drops down to -6 degrees.

Here I pressed lightly the accelerator a few times:

How could I diagnose this further?

Post by **mikealder** » 05 May 2014, 13:19

Look at the output from the accelerator pedal position transmitter as this could also cause similar symptoms to the ETM if it has started to fail - Mike

hss · Post by **hss** » 05 May 2014, 14:40

Thanks for the idea but I already checked that in Vida. And when it jerks while driving I can push the pedal further (maybe less than half) and it still jerks while under 2000 rpm. And even if it was a problem with the pedal transmitter I think it would never drop under idle rpm in neutral?

hss · Post by **hss** » 20 May 2014, 01:21

I have now replaced all coils and spark plugs, timing belt and cleaned the accelerator pedal sensor. I have tried it without MAP and MAF sensors and it still has the same symptoms. Can a faulty O2 sensor do something like this? Or is it the ETM circuit board?

hss · Post by **hss** » 27 May 2014, 08:58

Now I tried a different ETM but no change. Someone please give me an idea what to try next.

When cold starting the car the oil pressure light stays on for about 3 to 5 seconds. Could low oil pressure affect the VVT and cause problems? Usually a problem in the oil pickup tube seals?

hss · Post by **hss** » 30 May 2014, 12:34

Thanks for the idea. Replaced the cam position sensor but unfortunately it didn't help.

From a cold start the engine seems to work perfect for a while. Problems begin when the temperature needle rises near the center mark. But even when the engine is warm it idles perfect. Could it be an ECU problem?

It still takes a couple of seconds for the oil pressure light to switch off. Next we will replace the seals in the oil sump. Maybe a little bit low oil pressure can affect the VVT?

hss · Post by **hss** » 05 Jun 2014, 03:23

Seals inside oil sump replaced. Oil pressure is better now. There was a howling sound coming from the turbo when idling with a hot engine and maybe that was because of low oil pressure.

Replaced also the crank sensor but no help. We even tried to fool the engine temperature with a couple of resistors. Cold engine and 92 degrees celsius resistor, works pretty well. Warm engine and 20 degrees resistor = problems. So it seems that there is some component that does not like heat.

hss · Post by **hss** » 13 Jun 2014, 14:19

Now I have changed the crankshaft sensor and injectors but no help. It seems that the car has problems with low load. By that I mean the problems are most noticeable when trying to rev at idle (no load) and lessens by every gear change up (when driving). In 5th gear it is hardly noticeable. And if I push the pedal a little bit more, the car works just fine below 2000 rpm.

Is it the ECM? There is no cheap replacement? Any more ideas?

Post by **Atis** » 13 Jun 2014, 14:59

Check your LTFT, STFT and O2 readings, maybe your problem is only when you are in closed loop, indicating a possible O2 sensor problem. Your ignition timing is changing that harshly because the ECM tries to set the idle with timing.

Have you tried to clean the ETM? Maybe it is simply dirty, or stucks when it is hot (regarding ETM I am just guessing, I do not have experience with that)

hss · Post by **hss** » 02 Jul 2014, 07:02

Thanks for the suggestions. There's nothing obviously wrong with the LTFT, STFT and O2 readings. We cleaned the ETM while we installed the contactless TPS.

Now we have replaced a couple of vacuum lines and took the intake manifold off. Then after we installed the intake manifold (with a new gasket) the car was ok for about 25 kilometers. Problems started again after next start.

The car works ok when the engine is cold. Problems start when the temperature needle is nearing the center mark. Then if driven under 2000 rpm just touching the accelerator the car jerks maybe 3 times a second. Timing advance jumps between negative and positive values (after tdc). The throttle value is not oscillating. If given more throttle the engine works even under 2000 rpm. If I tease the engine like that under 2000 rpm for a while it will disable the exhaust VVT and idle starts to bounce. A restart fixes the idle.

I think it is because of the same issue that if I touch the accelerator at idle the revs drop. And if I keep the revs somewhere below 2000 rpm with a little more throttle the engine shakes and feels kind of like it was running on 4 cylinders (which it is not). If I don't touch the accelerator the idle is perfect.

We measured the voltage going to the CVVT solenoid. I know it is PWM but a regular multimeter seems to give useful results. The solenoid receives battery voltage and the ECM is grounding it. When idling it is something like 5 volts and if I touch the accelerator when the engine is cold it goes down by a couple of volts. But when the engine is warm and I touch the accelerator it drops immediately to 0 volts and works just like if the cable was disconnected from the solenoid. I don't know if this is normal or not. Should it be adjusting the VVT to it's max at those revs in any case?

Air leakage through throttle blade rises to a little over 12 when driving. It doesn't seem to be a problem though because there is no problem with the idle and there are no faults registered. I think I read somewhere that this should be less than 10.

And now the heater core started to leak...