P38A bad vibration in engine

**p38arover** · 23rd May 2007, 05:03 PM

Moved from another thread of mine:

Originally Posted by Scouse

That's nothing to do with me.

I swapped batteries but not engines !!

Let's hope it's not coolant leaking into the cylinder causing the misfire .

Originally Posted by p38arover

I've already replaced the engine a year or so ago.

It's not misfiring but it's down on power and vibrating quite bady.

I drove another P38A yesterday and it's smooth up to at least 5000 rpm. Mine vibrates at idle, then smooths out a bit then from about 2700 rpm it gets really, really rough like it's badly out of balance.

If it wasn't down on power. I'd suspect something else.

Last night I put an oscilloscope on each plug and the waveforms look much the same. If one cylinder had low compression, I think it would show up on the waveforms.

Nevertheless, I did a compression test today. All cylinders are with 5psi of each other. I don't know what the pressures should be but they were all 135-140 psi (the engine was cold - yeah, I know it should havbe been hot. I also forgot to open the throttle fully...... ). I didn't try them with a bit of oil down the bores.

I can't find the specs for the pressures on the 'net nor in the manuals but I suspect they should be around 160psi.

I can't imagine how the engine could go out of balance. It vibrates from idle upward. If the harmonic balancer had failed and rotated would that cause it?

Any suggestions would be appreciated.

Ron

**Scouse** · 23rd May 2007, 05:53 PM

How about running the motor with the serpentine belt removed ?
Just to isolate the ancilliaries before you start going too far.

**p38arover** · 23rd May 2007, 06:00 PM

Originally Posted by Scouse

How about running the motor with the serpentine belt removed ?
Just to isolate the ancilliaries before you start going too far.

Thanks, Scott. Yes, I had thought of that but was worried about coolant flow without a water pump. But for a few seconds of running it won't hurt. I might go out and do it now.

I'd drop in and ask you to drive/listen to/feel it it but I can't afford your company's charges.

I'm thinking of going to Graeme Cooper for a check-over. (I had been thinking of getting a UniChip conversion by Graeme - I was hoping to discuss it at the LROC Expo).

Ron

**Scouse** · 23rd May 2007, 06:15 PM

Originally Posted by p38arover

Thanks, Scott. Yes, I had thought of that but was worried about coolant flow without a water pump. But for a few seconds of running it won't hurt. I might go out and do it now.

I'd drop in and ask you to drive/listen to/feel it it but I can't afford your company's charges.

I'm thinking of going to Graeme Cooper for a check-over. (I had been thinking of getting a UniChip conversion by Graeme - I was hoping to discuss it at the LROC Expo).

Ron

You're more than welcome to drop by but I don't think I'll be much help. These sort of things are usually a process of elimination exercise.

Even a few minutes running would be OK with the belt off, especially if it's a cold engine.

**p38arover** · 23rd May 2007, 06:49 PM

Just tried it and the vibes are still there.

If I drop by, I'll ring first.

Thanks Scott..

Ron

**Scouse** · 23rd May 2007, 07:01 PM

I wonder if it could be an injector problem.

You mentioned you thought it had dropped a cylinder initially - do you think it's improved or the same ?

**justinc** · 23rd May 2007, 09:07 PM

i know they are crossbolted, but it sounds awfully like a loose or fretting main cap vibration...haven't seen a crossbolted engine do it yet though.

sorry, just a thought.

JC

**p38arover** · 25th May 2007, 02:28 PM

Originally Posted by Scouse

I wonder if it could be an injector problem.

You mentioned you thought it had dropped a cylinder initially - do you think it's improved or the same ?

It does it on LPG and petrol.

Re dropping a cylinder, I think I've disproved that with the compression tests. The problem seems to be getting worse.

I'm now thinking torque converter as it's the only thing heavy enough to throw engine out of balance at idle. But it doesn't get a lot worse at rtevs so that's confusing.

Ron

**p38arover** · 25th May 2007, 02:30 PM

Originally Posted by justinc

i know they are crossbolted, but it sounds awfully like a loose or fretting main cap vibration...haven't seen a crossbolted engine do it yet though.

sorry, just a thought.

JC

Hmm. I suppose I could check the cross-bolting easily enough.

Checking the mains is a little more difficult but not overly so. I never thought to check them the last time I had the sump off (a few months ago).

Ron

**rangieman** · 25th May 2007, 02:34 PM

a long shot but could it be the flex plate cracked or broken

years ago i had a falcon the torque converter came loose and that thing shook like hell

Thread: P38A bad vibration in engine

Thread Tools

P38A bad vibration in engine

Bookmarks

Bookmarks

Posting Permissions