C4 - Weak engine at idle - Update

[lancia58]

Hi

I have a C4 1.6 ( 110 bhp ) with AL4 automatic gearbox . It looks like in idle there is a lack of power. If I stop right next to a pavement curb and I try to get on the pavement , the car cannot go over the curb. When pressing the gas pedal after the car stops it hesitate till it accelerates. I suspected a vacuum leak so I hooked a Lexia and look at the various parameters especially the MAP reading and it shows at idle 540mb and it goes to 240 mb when revving to 4000 RPM. I notice that when I tried to go on a pavement curb ( can not go over it ) the engine can not go over 1600 RPM and the MAP shows 750mb. When it happens and I releasing the gas pedal the MAP reading stays 750mb for 5 to 10 seconds and it looks like the brake servo is not assisted by the engine vacuum and the travel of the brake pedal is much shorter than normal.
I replaced the MAP sensor just for the testing , I am not sure but it looks like it helps a bit.
I leave on a mountain about a 100 meters above sea level and the phenomena gets worth, in compare to driving at sea level. On top of it when trying to accelerate on a steep slope from standing at the beginning the car hardly moves. I didn't measure the fuel consumption but it looks like it increase. The car has 110,000 km.
Another thing worth mentioning is that about two years ago the pipes of the front window washers cracked and a lot of water spilled over the MAP sensor, and the check engine lit, and the idle was rough. When it happened I dried the MAP sensor and the idle gets back to normal, but from time to time the idle was rough and the check engine lit. Turn the car off and on sorted it, and after 6 months I t didn't happened again. A Lexia shows cylinder 4 misfire so I replaced the coil but it didn't help.

Any ideas how to sort this ?


Thanks
Zohar

[xantia_v6]

How does the car drive under other conditions?

I am not very familiar with this model, does it have an electronically controlled throttle?

Have you checked the transmission parameters? If the transmission is in limp mode, or if it is demanding torque reduction it could give symptoms similar to those you describe.

[lancia58]

Hi

But the problem I decribed the car drives OK and the transmission never goes to limp mode. It does have an electronically controlled throttle. What do you mean by "if it is demanding torque reduction".


Thanks
Zohar

[lancia58]

Any ideas please ?

Thanks
Zohar

[xantia_v6]

Hi
What do you mean by "if it is demanding torque reduction".

To produce smoother shifts and reduce strain on the transmission, the transmission ECU can request the engine ECU to reduce power by adjusting the throttle and ignition timing.

It is also possible that a seized stator in the torque converter will cause loss of torque when pulling away.

[lancia58]

Hi

Can it be that the electric throttle is not functioning OK ? If the stator is seized wont it affect the transmission at all speeds , and not only on idle ?

Thanks

[xantia_v6]

It is more likely that one of the ECUs is commanding the electric throttle not to apply power, for some reason (the live data from the engine and transmission ECUs should give a clue).

A seized stator in the orque converter (although rare), mostly affects the torque when pulling away from stationary, because that is when maximum torque multiplication should occur, it has no effect above 2000 RPM.

[lancia58]

Hi

By using a Lexia can I identify a seized stator ?

Thanks

[Mandrake]

It is more likely that one of the ECUs is commanding the electric throttle not to apply power, for some reason (the live data from the engine and transmission ECUs should give a clue).

A seized stator in the orque converter (although rare), mostly affects the torque when pulling away from stationary, because that is when maximum torque multiplication should occur, it has no effect above 2000 RPM.

Funnily enough I think my old V6 Auto that had gearbox problems and eventually a failed gearbox may have had a problem with the stator in the torque converter (I speculated about it at the time) as it did not have the the initial "rush" of acceleration you would normally feel when accelerating from stationary, and the difference to the current V6 I have now was very obvious when directly comparing them - it does have this automatic gearbox like rush of initial acceleration.

Hi

By using a Lexia can I identify a seized stator ?

I can't think of any way you could diagnose this with a Lexia unfortunately. All the ECU knows is torque converter input speed, (engine speed) torque converter output speed ("input speed" to the main epicyclic gear train) and calculated engine torque before the torque converter.

When a stator seizes the torque converter no longer provides torque multiplication (up to 2x typically) and instead acts as a simple "fluid coupling", but the result of this is mainly less torque into the main epicyclic gear train than expected during initial takeoff. I don't think the ECU can detect this as it has nothing to measure torque after the torque converter.

https://en.wikipedia.org/wiki/Torque_co ... lure_modes

Hi

I have a C4 1.6 ( 110 bhp ) with AL4 automatic gearbox . It looks like in idle there is a lack of power. If I stop right next to a pavement curb and I try to get on the pavement , the car cannot go over the curb. When pressing the gas pedal after the car stops it hesitate till it accelerates. I suspected a vacuum leak so I hooked a Lexia and look at the various parameters especially the MAP reading and it shows at idle 540mb and it goes to 240 mb when revving to 4000 RPM.

540mb at idle seems pretty high to me. That is 15.9" of mercury. (HG)

https://www.brandtech.com/brandtech-vac ... -pressure/

Idle vacuum for petrol engines is normally between 17" and 21" HG so I would say that is out of the normal range. And only engines with very low idle speeds (such as V6/V8) would have a normal idle vacuum as low as 17", smaller faster idling engines would be more like 20".

What vacuum do you see if you hold the engine at 1000rpm ? Low vacuum might be a vacuum leak, but that would not cause you a low power issue when you open the throttle wide so it may be a problem but unrelated to your curb climbing lack of power.

I notice that when I tried to go on a pavement curb ( can not go over it ) the engine can not go over 1600 RPM and the MAP shows 750mb. When it happens and I releasing the gas pedal the MAP reading stays 750mb for 5 to 10 seconds and it looks like the brake servo is not assisted by the engine vacuum and the travel of the brake pedal is much shorter than normal.
I replaced the MAP sensor just for the testing , I am not sure but it looks like it helps a bit.

Hang on - after you release the throttle and the engine drops back to idle, the MAP reading stays as high as 750mb ??? If that is a true reading then you could be looking at a severe exhaust restriction. A severe exhaust restriction due to for example a melted cat, or a broken up cat that has blocked downstream piping would cause the idle vacuum to be unusually low, and would also cause pressure to drop slowly when you released the throttle and returned to idle. If you suddenly close the throttle on a normally running engine the pressure should drop instantly to a very low value like 200mb. A severe exhaust restriction would also cause a big loss in power.

A Lexia shows cylinder 4 misfire so I replaced the coil but it didn't help.

How long was the car running with a misfire ? I don't want to panic you but if the car was driven when down on power for a long time and it turned out there was a misfire in one cylinder due to a coil failure you could have a melted cat causing an exhaust restriction - even if the misfire is now fixed.

Any ideas how to sort this ?

Based on the extreme lack of power, unusually low vacuum reading at idle and after releasing the throttle, and the possibility that the car was driven for a while with a misfire, I think your first step is to do an exhaust backpressure test. This is done typically by removing the upstream oxygen sensor and connecting a suitable pressure gauge in its place. Adaptor fittings to do this are available. (I still have a kit to convert an oxygen sensor hole to a hose for a pressure gauge) ScannerDanner has a couple of excellent videos on doing exhaust backpressure testing, here are a couple that you should watch:





I followed his testing procedure on my old Xantia V6 when I suspected it had a blocked/damaged catalytic converter, but it passed the test and there was no issue with the cat in my case.

[lancia58]

Hi Simon

Thanks for the detailed answer. I defiantly will check the back pressure


Zohar

[lancia58]

Hi Simon

Today while parking I pressed the gas pedal all the way till 5500 RMP I felt a sort of "Bang" from the exhaust and than the RPM dropped till the engine stopped. I waited for couple of minutes and tried to start the engine, it was hardly started and it was very rough and didn't go above 300 RPM. I think that as you said the cat was clogged and the high RPM broken it and blocked the exhaust. Since it happened late in the evening I could not do any thing but guessing. Is what I written above makes sense ?

Thanks
Zohar

[Mandrake]

That's a shame Zohar, it does sound like a faulty cat.

As you say, most likely when you revved it hard a chunk of the damaged cat honeycomb broke free and was blown downstream through the exhaust until it lodged in the muffler or a bend in the pipe mostly blocking the exhaust flow.

If the car can barely start and run at all now there is probably no point doing a back pressure test. You're going to have to remove the exhaust system from just in front of the cat right to the rear tail pipe so you can inspect the cat and the muffler and intervening pipe to find and remove any debris.

Depending on the cat design if it has a joint near the output of the cat with a straight section of pipe you should be able to look directly into the output side of the cat with a torch once disconnected from the tail pipe - if you can see a piece of the honeycomb is missing and/or it looks melted you know you've found your problem.

Fixing the muffler blockage shouldn't be too hard (maybe the muffler will need replacing) but if you are replacing the cat you need to be sure that any problems such as a misfire that originally damaged the cat are fixed so it won't happen again.

[lancia58]

Hi Simon

I think the source for the problem is a cracked hose/pipe of the window washer jet. Two years ago while driving I washed the window with the jet and suddenly the engine revving became rough. I stopped the car and opened the bonnet. I found that one of the nipples of the washing pipes broke and the washing water spilled on the engine top I guess on the MAP sensor and other components, and the check engine lit. I wiped the water and waited for couple of minutes and all was back to normal. Than from time to time ( quite rare though ) the engine revving became rough and the check engine lit. I guess it caused the misfires that in turn clogged the cat convertor.

Thank
Zohar

[lancia58]

Hi

I found today that the connector that is flaugged onto the coil pack disconnected. I guess it wasn't plugged all the way after a mechanic checked it, and due to vibration when the engine revved high it slipps out from the coild pack.

Reverting to the original problem, I found that when the engine starts the alterantor charging volatge is 14.2v that is OK , but after the engine gets warm it droops to 12.6v that it too low and at the same time the idle becomes rough. I also checked for the alternator ripple with AC voltmeter ( I guesse it is coupled with a capacitor to block the DC component ) on start it shows 20mv RMS that is normal but as the angine gets warm it shows 120mv RMS that is too high. So I think the alternator is faulty and it may be the source for the low engine power at idle.

Thanks
Zohar

[lancia58]

Hi

Well I had the time and drove the car to Auto Gearbox specialist and he diagnosed that the torque converter is failed.

That may explain the problem of the hard brake pedal when the engine is under load see the following thread

http://www.frenchcarforum.co.uk/forum/v ... =3&t=55836

Since the torque convertor is failed there isn't any torque multiplication when engaging the gear to Drive the load on the engine is more in compare to a car with a good torque convertor. Now in order to keep the idle RPM more fuel is needed ( to overcome the gear resistance ), more fuel needs more air , more air is less vacuum hence harder brake pedal.


Regards
Zohar