Rough idle and acceleration

Hi all

I have a 1997 4.0l petrol P38 and I have a problem which is driving me crazy.

The car run fine until the recent wet weather but has since developed an annoying fault. The car was used through many deep puddles but never taken through extreme water levels etc.

The car will idle, but a little lumpy. It also hesitates when the accelerator is pressed and vibration can be felt through the entire vehicle if you accelerate from any speed. It feels like its missing on a cylinder????


Are there any common things which can cause this?

BTW, the car is well serviced with new plugs leads etc and has only developed the fault since the wet weather.

Thanks in advance

down on a cylinder or it has been known for coil packs to fail from what you say

You have got to start with the basics. Check each plug for a strong spark, compression test each cylinder while you are at it. If it is a coil pack it will be pretty obvious with the testing.

I've just checked all the plugs and a strong spark is present.

I was going to get it plugged in to see if any fault codes are showing but the bloke said that unless the check engine light is lit nothing would show. Is this correct?

Something else I should mention is the car is gas converted.

The fault is exactly the same whether on gas or petrol.

Could it be an injector problem? Does it rev happily?

Revs fine, just a bit of stuttering on pickup.

Would not think its an injector as with both petrol and lpg the fault is the same.

That points to ignition, timing or fueling....

Best get it on a diagnostic and check the live readings or if there are any stored fault codes...

could be the dual mass flywheel that can make them idle rough and loss of power

Just had it running in the dark. 2 x ht leads were flashing like mad.

Hopefully this is it, I'll change and post the result tomorrow.

Ok, replaced the leads and plugs - still the same

It seems as though the ignition is not advancing when the accelerator is pressed as once it catches up it revs fine???

I,ve just had it plugged in for diagnostics - No faults found @@@@

The bloke did say though that when revved, the airflow meter readings did not change??

Should the airflow meter readings change when revved or does anybody have any more ideas??

yea defo should, could well be the reason it runs like crap

How's your air-filter, wet?

Air filter is dry.

If you unplug the maf it trys to cut out, if you give it some revs when unplugged it stabilises but still runs the same as when plugged in.

Been told that if the fault is the same on the gas it's more than likely going to be the coil pack, as the maf is not utilised as much during gas use?

Should have a replacement coil pack to try tomorrow so fingers crossed.

Monkeymut said:

Air filter is dry.

If you unplug the maf it trys to cut out, if you give it some revs when unplugged it stabilises but still runs the same as when plugged in.

Been told that if the fault is the same on the gas it's more than likely going to be the coil pack, as the maf is not utilised as much during gas use?

Should have a replacement coil pack to try tomorrow so fingers crossed.

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Why would the MAF not be used on gas? It measures the mass of the intake air which is used by the ECU for adjusting the mixture, gas needs that just as much as petrol as far as I know.

Tried both a replacement maf and coil - still the same.

We have however noticed that if you disconnect the ht lead from either of the two front cylinders on the left bank (as you look at car from front) the car runs the same?? both plugs have been checked and are sparking so we are now thinking it may be the head gasket gone between the two cylinders.

I'm going to compression test it tomorrow so we'll see.

If you have a good spark on those two plugs but it makes no difference when they are disconnected, sounds like no combustion in these cylinders, fuel delivery issue?

Stufer said:

If you have a good spark on those two plugs but it makes no difference when they are disconnected, sounds like no combustion in these cylinders, fuel delivery issue?

Click to expand...

or a spark plug failing under pressure?

Just checked compression on all cylinders.

6 reading at 150psi and cylinder no's 2 and 4 reading 50psi - head gasket it is then!