4.2 running rough at light load

[RobO999]

Gents,

Would be grateful for some advice, I have a 96 4.2, at light throttle its running somewhat roughly, not really a misfire, but feels more like erratic fueling (may be wrong of course as I have no real experience of the v8). Its most noticeable if, for example. trying to maintain a constant 40 in 4th gear where the engine won’t hold constant speed. The whole thing gets worse as it gets warmer in traffic (about 90 on temp gauge).

From reading the archive I suspect the throttle pots, or throttles themselves but also wondering about MAF and or temperature sensors?

I also read that it’s possible to reset the controller, how can I do this? does this reset the adaptive maps etc? And is it likely to help at all.

I live in the NW near Preston, so any recommendation on dealers or specialist would be welcome. I bought the car a 3 months ago from TMS, it seems to have got worse, but the weather is warmer now too. It’s a bit of a treck down there for something which may be a straightforward issue.

Thanks,

RobO

[simonsparrow]

If you MIL light isn't on, then in theory the sensors are ok. Only the ECU software can tell.....

Otherwise, how about this? www.pistonheads.com/gassing/topic.asp?t=69466&f=6&h=0&hw=jerky+travel

[andy4200]

They are a bit like this. Mines was the same.

It has the Joolz mod mentioned on the other thread and after Austec serviced it a couple of months back it has been very smooth since then.
Mine does seem to get wworse though the closer it gets to service point, might be trying to tell me something

[kojak69]

I would certainly check the throttle pots first. These are cheap to replace. Mine was running similar to yours, and the throttle pots were the main culprit. Also had new HT leads as well.

I assume you use super unleaded, and if posible, Optimax. Mines runs better with the latter.

[Jo Williams]

Could also be the throttle linkages, if they are out of whack, one bank will be getting more/less fuel than the other I think.

Mine is the same, always fine after a service, gradually gets worse.

Plugging the laptop in and looking at the readout will tell you what values are on each at the throttle pots, and will at least tell you if they don't match. A re-set of the maps may help a bit too.

Jo

[trooper1212]

Lambda sensors can also be the culprit, mine have a tendency to throw faults every so often and kick the car into lumpyville, which is a little annoying. Especially as the last time they went the fix was to replace the ECU and sensors with new ones, find the fault was still there and put all the old stuff back, for the fault to have then disappeared.
Cheap fix, but annoying none the less, it must be a dodgy connection somewhere.

[RobO999]

Thanks for the info.

Can anyone confirm if its possible to reset the controller, i read several versions, some say power it down for an hour, some say needs the dealer software.

Will look at the linkage tonight and also the output from the throttle pots on a 'scope just to see if theres anything glaringly obvious.

Also any recommendations for someone in the NW. Either dealer, specialist or someone with the software.

Cheers,
Rob.

[richardp]

I had similar symptoms which I thought was electrical so went through the process of eliminating the plugs, coil packs, throttle pots etc.

In the end I gave up and took it to Andy at APM who diagnosed it as the clearance being wrong on no.7 exhaust valve simply by listening to it at idle. Sure enough, it was and after he performed his magic it is running very smoothly now.

Richard