GQ TB42e - mystery"check engine" prob - Not at my wits end - yet.

Hi all,

My 96 GQ LWB with the TB42e injected motor dual fuel has gone from running well to being almost undriveable in the space of a week - and I am plagued by the "check engine" light.

The story so far:

Begins to run rough a week ago - won't idle smoothly and surges at idle from 600 rpm to 1000 rpm and back to 600 - like blipping the throttle. Stumbles and almost conks out as revs fall. I can drive on gas if I keep the revs up around 2200 - 3500 but at the traffic lights it sounds like I am looking for a drag race on 5 cylinders. Will NOT run on petrol - one cylinder only, backfiring and loads of smoke.

So I begin to check / swap out the different sensors. I reviewed the different threads on this site first though.

The Oxy sensor on the exhaust looks like it was the one installed by the factory - swapped it out for a new one - no change. "Check engine" comes on when it stalls.

The Coolant sensor looked ancient - swapped it out - no change. "Check engine" comes on when it stalls.

The MAF sensor looks very expensive, so I unplugged it while the car was running - no effect on the motor - and I guessed that the motor was in "limp" mode. Cleaned it with the right spray, put it back, started car, "Check engine" comes on when it stalls. Swapped with known good unit - no change.

The crank angle sensor in the distributor was next in the firing line - swapped it out too - cleaned up distributor at same time. Now it runs without big surges, but only at 600 rpm and still runs on 5 cylinders. It can be revved up but the miss is there and the exhaust note sounds like a tractor (on gas) and worse on petrol - stinking up the area with unburnt fuel and running like a dog.

There is a pool of water collecting under the exhaust outlet, drips of condensation from crappy combustion and mixed with black sooty stuff. I checked the coolant level each time I fire the car up and there is no loss from there, which appears to rule out a coolant leak into the cylinders.

Back to the manual and testing the impedance of the power transistor and the throttle position switch shows they are fine. Do the same to the air valves at the back of the plenum chamber - they are good too. All separated electrical contacts get a dose of contact cleaner, just to be sure.

So - I pulled the spark plugs to look at them and they were dry, black and sooty. Cleaned and gapped them - Put it all together - still runs like crap.

Double checked all air hoses to rule out air leaks downstream of the MAF - all were OK.

About the only thing I haven't done is swap out the ECU.

Can anyone suggest anything else?

yer one tip either get a mechanic to scan the ecu with a scan tool or lay your hands on an ecutalk so you can scan them your self. otherwise you just gonna keep throwing parts at it hoping to find the problem

Yeah, I will get that ECU scanned and stop splashing money around at Bursons. Thanks billyj.

Go back to basics,Hows the timing??
Have you checked the in-line cage filter on the fuel line.
Could it be dual fuelling?? check your coolant level as well

Non expert gut feeling ----- air leak somewhere.

Thanks everyone - I have been busy on the issue. Took Cuppa's advice and went over the engine, checking any sus looking hoses on the vacuum side of things. Some of those things were hard and had no flex at all - no doubt been there since the factory. I got hold of an ECUtalk which says....Air flow meter. Hm - that $50 "known good unit" MAF was a waste of cash. So I have had to cough up the cash on a new air flow meter (ouch). WTF - why are they so expensive? Lets see what happens next...

Threedogs, how does dual fuelling happen?

Be careful when fitting an Air Mass meter. If the engine backfires it is quite possible to destroy the new unit from the extreme pressure coming back through the ducting. They are tricky to diagnose unless completely failed which sounds like yours is.

If the air mass replacement is unsuccessful I would be definitely be checking the valve timing as that fits all the symptoms you are describing. Blown head gasket also is something to consider. As you may know, there are many ways a head gasket can fail. One really tricky one is where it burns between two cylinders which causes all sorts of funny things to happen while the engine is running.

Yes, it's not the MAF - one expensive and brand new MAF = no change to what's going on. To start the motor on gas involves putting the accelerator pedal about a third to half way to the floor while turning the key - otherwise it will turn over forever on the starter without catching. Petrol is even worse. I am looking under the plenum chamber on the inlet side and can see some kind of valve under there, with an electrical connection and mystery pipes. The Gregorys manual doesn't mention it other than in the wiring schematic where it is referred to as the "air regulator". I don't know what that is for but it is about the last thing I haven't replaced / cleaned and it is in a bastard of a spot - any clues anybody?