The V8 Owners Forum -

OK, my P38 is exhibiting the most peculiar anomaly:

It's a 4.6 with OMVL Dream 21 LPG.

Runs perfectly on petrol. Runs perfectly on LPG at light to moderate loads. But:

When under heavy load between 3500rpm and 5000rpm, it judders pretty badly. Perhaps not as regular and harsh as a total misfire, but very very jerky nonetheless.

I've done some diagnostics with the LPG software and an OBD-II reader, and the following are true:
1. The Injector times go wild when it judders
2. The MAF reading goes wild (is 80-90 when on petrol, jumps up to 270 when on LPG!)
3. TPS is steady

I can't understand why it should be perfect on petrol, yet not OK on LPG.

Any ideas?

Thanks,

Chris.

Hi Chris

have you looked at the HT leads for tracking , mine old one used to do that on LPG but not on petrol ..it was tyhe HT leads breaking down as they coped with petrol but not Gas ...

ChrisJC wrote:OK, my P38 is exhibiting the most peculiar anomaly:

It's a 4.6 with OMVL Dream 21 LPG.

Runs perfectly on petrol. Runs perfectly on LPG at light to moderate loads. But:

When under heavy load between 3500rpm and 5000rpm, it judders pretty badly. Perhaps not as regular and harsh as a total misfire, but very very jerky nonetheless.

I've done some diagnostics with the LPG software and an OBD-II reader, and the following are true:
1. The Injector times go wild when it judders
2. The MAF reading goes wild (is 80-90 when on petrol, jumps up to 270 when on LPG!)
3. TPS is steady

I can't understand why it should be perfect on petrol, yet not OK on LPG.

Any ideas?

Thanks,

Chris.

Any EFI system depends on a reliable tach signal from the ignition. And a good clean supply of battery volts, including the ground. Corruption of either could cause your problems

My guess is the tach signal. Have you a method of logging it?

Thanks chaps.

The 'tach' signal is OK. I know this because the rev counter is stable on the gas diagnostic software.

I wondered about the leads. I'm reluctant to fork out for a new set without proof, as they're dear for a Thor engine, and they are less than a year old. Mind you, they were 'pattern' leads rather than super-duper expensive ones. Rimmers want £101.22 for a genuine set!!

I think I'll strap a video camera under the bonnet and go for a drive. See if there's any evidence of tracking.

Chris.

buy some copper core cable Chris and pull end off an old set of leads you can then solder them back on the new wire ( don't forget to put the caps on first though ...i forget )

I have ordered a set of super-duper (not Magnecore!) leads from eBay...... Shall see if that makes a difference.

Chris.

fingers crossed Chris , when i went to TINLEY TECH a while back he said that even the OE leads only last 6/12 months tops on Range Rover with LPG ... they just eat them appaently

FIXED!

Turned out to be the MAF.
I'd fitted a £40 eBay pattern MAF. In desparation I bought a genuine part from Rimmers (£140), and it's perfect!

I had tried 3 sets of leads, to no avail in the meantime.

Chris.

Glad to hear its sorted , things like thsi can be a right pain to find ....