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?

Have you checked your crank angle sensor, ignition timing, throttle position sensor?

Is the ecutalk still saying MAF?

Have you reset the ecu?

If you disconnect the MAF does the engine run the same, better or worse?

Have you checked the wiring from the MAF back to the ecu?

Quote:

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Originally Posted by Luddite

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.

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More non-expert thoughts to consider.
?identify which cylinder is the one which is missing. First try swapping over ignition leads to see if affected cylinder 'moves'. If leads (& distributor) are ok I'm wondering about the possibility of an exhaust valve not closing properly? Maybe a compression test?

have seen a tb45 drop a push rod. valve stems gummed up

Hi Irish,

I replaced the crank angle sensor, checked the timing using a light at 10 degrees BTDC, checked the TPS with a multimeter - it is within specifications and advances / reverses appropriately when the throttle spindle is turned to fully open and back to closed.

ECUtalk still says MAF. On the main screen it says that the MAF is sitting at a fraction over 5 volts.

I reset the ECU by disconnecting the battery for about half an hour.

I checked the compressions in all cylinders - between 1000 and 1040 kPa in each - I was trying to exclude a leak between cylinders and they all appear fine. Needle climbed steadily in each cylinder test (3-4 turns of the crank).

Some small change when the MAF is disconnected - it runs slightly better?

I started it on gas and held it with the hand throttle at around 1000 rpm.

I noticed that disconnecting plug leads 2 or 3 while the motor was running made no difference to the poor running. Disconnecting 1, 4,5 or 6 produced a noticeable change in running. Leads 2 and 3 threw off good bright sparks between the dizzy cap plug hole and the metal plug end as they were removed. I ran it in the dark and can't see any sparks sneaking out of the sides of the leads.

I runs really poorly on petrol. I am beginning to wonder about the ECU. As I understand it, the ECU makes decisions about the amount and timing of fuel to be injected based on info from other sensors. When it is on gas, the engine runs "better" than on petrol - where it really bogs down and the smell of raw petrol is strong.

I pulled the ECU and took off the covers, but there was nothing to see there - no corrosion, no loose crap.

Well - I replaced the ECU and hey presto the MAF voltage on the ECUtalk is rising and falling depending on the throttle movement - where as before it was sitting up on 5.115 volts and not moving. Now it runs - better - but still has issues. As it idles it still sounds like it is dropping out a couple of cylinders. As I rev up it still has a rough miss sound too.

If the head gasket was leaking between cylinders - say 2 and 3 - should compression be poor? It seems like it's having trouble firing on all cylinders - can a blown head gasket interfere with ignition?

By process of elimination and getting a fresh set of eyes on things, it looks like the fuel pressure regulator has crapped itself and the injector "baskets" were clagged with E10 petrol "varnish". The regulator was changed 3 years ago before a trip into the desert "just to be sure" and now appears to be letting neat petrol straight into the inlet manifold. I have dug out the old one and will switch that back in to see how we get on. Three Dogs was on the money with the "dual fuelling" guess - all the black sooty crap was to do with petrol AND gas getting into the manifold at the same time, and not getting properly burned up. The clue was the motor ran fine on gas with no petrol supplied to the injector rail. Once petrol was put back to the rail, the crappy running happened again. Initially, blocked injectors leaking down was suspected. I learned that the injectors on dual fuel TB42e motor are "switched off" when on gas but this doesn't stop the fuel pump running, pushing fuel to the fuel pressure regulator and the fuel rail. When on petrol, the injectors "flutter" making the petrol mist in the inlet manifold. A pressure check of the injectors showed they were clagged up - but not leaking. The internal filter baskets were coated with "varnish" from ethanol mixed petrol preventing them from working properly anyway. It seems that E10 fuel is bad news for dual fuel EFI motors that only use petrol now and again, as the ethanol part sticks to the inside of the injectors and stops them working properly. They have been cleaned professionally now. It's fingers crossed that the regulator switch over will fix this issue.

Still sound like cyl 2&3 are doing something wrong.
Ecu swap won't fix that as its a distributor fired engine.
Dissy, Rotor, Plugs, leads, cap, all prime suspects, cleaning and gapping time!
Plugs are cheap, even swapping cyl see if miss moves if money's tight.

E10 is bad news for most engines of older cars. i avoid it like the plague.
try to burn one tank of petrol for every 2 of gas, should help avoid the clogging up.

Thanks for the advice along the way everyone. The GQ TB42e lives again -the fuel pressure regulator was the cause of the fuel prob.

So to recap, terrible starting and running on gas and even worse running on petrol was caused by a fuel pressure regulator on the fuel rail developing a fault and tipping neat petrol into the inlet manifold. This meant that on gas, the engine was getting gas AND petrol at the same time.

The clue was black sooty crap coming out of the exhaust and also coating the plugs. Disabling petrol delivery meant we were good to go on gas, isolating the problem to the petrol side of things.

Terrible running on petrol was because of poor pressure in the rail (cos the regulator was letting the fuel go elsewhere) and the injectors were clagged up with E10 fuel crap.

Lessons learned include: pay attention to the obvious - loads of black soot and the stink of unburnt petrol. I should have started there. I got sidetracked cos the regulator is basically brand new and I just didn't think about that being involved. just proves new parts can also fail quickly.