Weird revs-related power loss on 2.1

Post by **syncroandy** » 29 Aug 2006, 18:39

1990 Syncro 2.1 MV Digifant, running sweet since I got the van in 2003, apart from occasional mild Vanagon-syndrome which I haven't been bothered enough about to deal with (eg. harness or capacitor fix). OMVL Millenium conversion fitted Nov 05 along with new Bosch Lambda probe. LPG running sweet for 5-6k miles. I start up on petrol then the system switches to LPG after passing 3000rpm for the first time.

The other day immediately after starting off I experienced a noticable power loss in second when the revs were passing from 2-3k, after a short trip on LPG, I noticed the same problem. As the revs are increasing there's a sudden loss of power (and drop in revs) which quickly clears if the throttle is held open and the revs recover past this and continue up as normal. This only happens on petrol and not on gas.

My first thought was a split in the inlet manifold opening up as the engine moves, leaning out the mixture and causing power loss. However this would happen whether on petrol or gas, I checked the main rubber manifold anyway and couldn't see any splits.

The next candidate was a failing AFM (eg. worn or dirty track). At Aidan we swapped-out the AFM for another, with no improvement.

I then tried resetting the Digifant ECU by disconnecting the battery. No improvement.

As Aidan had a spare ECU, we swapped that. Again no result.

We then put on my spare brand new Bosch distributor cap and rotor. No result. It this point we admitted defeat, and I went on my way.

Tonight, I've tried disabling the LPG system (disconnected power and bypassed the injector-disabling relay. No result.

Finally I can report more details on the symptoms. Running on petrol, from cold, at any revs above about 2000, it will only sustain steady revs for about two seconds before dropping down to below 2000, and then bounce up again. It will sustain this oscillation for as long as the throttle is held at the same position.

Any ideas gratefully received !

Andy

Post by **Simon Baxter** » 29 Aug 2006, 20:19

If the fault happens on petrol and gas it is unlikely to be a fault with the fuel injection side of things as the fuel injection is effectivley turned off while gas in enabled, if it happens on both then it is likely to be something that is used on both systems, i,e the ignition system.
Have you swapped the coil? dizzy? Plugs?

Post by **Simon Baxter** » 29 Aug 2006, 20:20

Oh, and check the servo pipe, have a spray around with the WD and listen for changes in engine note.

Post by **syncroandy** » 30 Aug 2006, 09:16

Cheers Simon, as I said, the fault's on petrol only not gas, so I should be focussing on the injection side of things.

Tonight I'll try disconnecting the lamba sensor, if it's failed in an odd way than that will be thowing off the ECU. Odd that the symptoms only happen over 2000rpm. though.

Post by **Mocki** » 30 Aug 2006, 11:21

petrol pump or filter?
i remember a inj doka with sand in the tank did this, blocking the filter shortly after start up and revs dropped after reaching about 2k then it cleared untill start up next time, turned out there was sediment in the filter housing from the contaminated tank......

Post by **syncroandy** » 30 Aug 2006, 11:57

Cheers Mocki, possible fuel starvation is certainly worth checking.

It'd be nice to check the fuel pressure on the injector side of the regulator but I don't have the right guage. It may be worth taking to a local shop to have this checked.

If needed I have a spare fuel filter and also pump. I've never had the fuel system apart on an injection vehicle, which are alleged to hold the fuel under pressure. I have visions of litres of fuel squirting all over me when I open it up. How big a deal is this ?

Post by **Russel** » 30 Aug 2006, 12:15

Check that ure earths on the engine block and in engine bay are making good contact.