Stumbling V8!

Hi guys,

I've put a post in the P38 section, but thought I'd ask in this section as well if it's just a V8 issue.

The P38 has been behaving itself for a while now but my stumbling problem is back.

It started off fairly gently as just a very occasional sudden drop in revs, but it worsened over time to the whiplash inducing stumble and surge buckaroo and stalling when stationary.

I'd assumed this was down to blocked fuel filter/crap in the tank, as it only seemed to start when the fuel was getting low and changing the fuel filter seemed to cure the symptoms.

I drained the fuel as much as I could and put a new filter on again, but the prob is still there.

Apologies for similar posts if diff sections but any suggestions of causes/cures will be much appreciated (apart from torch it! )

not too expert on p38's but I had a bmw that did that, every garage under the sun tried to sort it with no luck. I sorted it one day, found a wire to the coolant temp sensor, wiggled it and the reves went up and down.

spliced in a new connector and a new sensor just to be sure and all was well.

worth checking anyway

Hi there. Sorry to hear you got running problems. The first stop would be to get your ECU codes read. Could be several things, but once you have to fault codes you'll stand a better chance of locating the fault n fixing it without spending loads replacing unneccessary stuff.
good luck

Hey Joe,

Cheers for the reply.
I've got a bluetooth ODB reader which lets me get at least some of the data from the P38, I keep an eye on the error codes and the ECU registers nothing.

Am always reluctant just to replace bits without knowing they're the cause, half the time cos they're bloody expensive and the other half cos I'm a cheapskate!

Have a few possible suggestions to look at from fuel cap, IAC valve or the evaps system.

JD. said:

Hey Joe,

Cheers for the reply.
I've got a bluetooth ODB reader which lets me get at least some of the data from the P38, I keep an eye on the error codes and the ECU registers nothing.

Am always reluctant just to replace bits without knowing they're the cause, half the time cos they're bloody expensive and the other half cos I'm a cheapskate!

Have a few possible suggestions to look at from fuel cap, IAC valve or the evaps system.

Click to expand...

It should come up with atleast one code after it has stumbled and/or stalled. But you have to read the code during or directly after the event. With the RRC's I've messed about with the codes have a habit of clearing themselves until directly after a repeat problem happens again

Cheers for the tip joe, will try and be a bit more active with the timing on the code reader!

Ok start at the top simple silly questions
1 does it happen when hot or cold
2 when is it at it's worst? Hot or cold?
3 when was it serviced last?
4 does it have a gas conversion?
5 Is it worse when you have low fuel in the tank?

vougese39 said:

Ok start at the top simple silly questions
1 does it happen when hot or cold
2 when is it at it's worst? Hot or cold?
3 when was it serviced last?
4 does it have a gas conversion?
5 Is it worse when you have low fuel in the tank?

Click to expand...

Yes, its all a process of elimination.
Codes can help start you in the right direction, but its not reading defo go with the simpler/ less expensive options first. Starting with the questions vougese39 posted above is a good start.

Incidentally, not saying it is this, but the previous owner of my silver RRC daily driver said he had similar problems when the engine had warmed up. Turned out to be the ignition module cooking itself. He bought a replacement and fitted a bigger heat sink and spaced the module away from the distributor body and it fixed the issue.

1 does it happen when hot or cold
Both really. Though not all the time.
2 when is it at it's worst? Hot or cold?
Seems to be more noticeable when hot, but obviously it gets up to temperature pretty quick.
3 when was it serviced last?
Have serviced it myself recently (plugs, leads etc)
4 does it have a gas conversion?
Not yet
5 Is it worse when you have low fuel in the tank?
Seemed always to be kicked off by the tank getting low.

I've ordered a new fuel cap and found a reasonable IAC valve, will see if these two make any difference.

I would guess if you have a full tank and no issues then it is the pump pipe to the top of the tank and you will need a new tank unit very common more so in the diesels but have done a few petrols over the years.

As the p38a has coil packs the leads need to be invert sercure if any of the leads are loose then that could be the issue but it would put a cylinder miss fire fault only problem with most generic fault code readers they will only give you ecu faults and limited or no parameter readings or live data which does restrict diagnostic.
Have you got a petrol pressure gauge connect that and see what it is kicking out

I replaced the fuel pump a while back...not got a proper fuel pressure gauge, used an air pressure gauge and the pressure looked right (rising on acceleration).

Have been putting off getting the LPG conversion because of this issue...I know it's probably not the smartest thing to do but I'm giving serious thought to getting the kit just so I can isolate the damn fuel system!

As you have not isolated the fault fitting another fuel system will not help as you will still have an issue.
Are you sure it is not a mechanical issue or fault?

It could be mechanical, just didn't think it was very likely because it's been quite random...though it's bucking most of the time now.

It does it in all gears and stalls in neutral too so didn't think it was a gearbox issue...any other ideas to help isolate the fault?

Had a bit of a disaster with a ruptured brake line so the car's gonna be off the road for a bit.

I have a similar problem with my 1996 P38 4.0 V8 petrol/LPG.

When it gets warm it sometimes misfires or cuts out completely and then there is often a problem restarting. This happens whether it's running on petrol or gas.

I tried changing the cam sensor, as several mechanics suggested this could be the cause, but it still happens, although so far to a lesser extent, ie. not as often.

If anyone has any suggestions I would be very grateful.

cheers
Gordon

Ecu test and live data time you need to see the coil recharge times injector pulse widths and the cylinder imbalances too.
Compression test would be good to see if it is a sticking valve or a worn cylinder