P38A Compression Test Results.

MrGorsky · May 7, 2020

Morning Everybody.

My misfire under load is still apparent, so I've bought new plugs and other bits to see if that helps. Whilst changing them I did a compression test and these are the results I got.... with my cheap eBay tester.

1=125 2=130
2=130 4=145
5=125 6=155
7=130 8=140

All figures in psi. Does that look about right for an engine with 158687 miles on the clock?

I thought I had a blowing exhaust from the front and have bought new cats, but having taken it all apart, my old original LR cats whilst rusty are pretty good. I don't think there's a hole there.

I don't want to put it all back together if there's serious trouble brewing elsewhere, so I thought I'd ask for advice on here first!

kermit_rr · May 7, 2020

i'd say 125 is a bit low, but still acceptable, while they'd ideally be closer, they are all within a reasonable margin

MrGorsky · May 7, 2020

Thanks @kermit_rr that's very encouraging. At least my head gasket isn't toast......yet.

Looks like the engine rebuild can wait another couple of years then!

kermit_rr · May 7, 2020

a small amount of oil down the bore just before you do a compression test will tell you whether its leaking piston rings or not. could also be valves

Henry_b · May 7, 2020

That compression is low for a Rover V8..

A tired RV8 should be atleast 150psi..

the difference between #5 and #6 is concerning a difference over 10% requires investigation..

Mine was between 190 and 210 psi...

drop some oil down the bores and do it again, thottle wide open..

kermit_rr · May 7, 2020

Which cylinder(s) misfire? Have you checked the camshaft is ok?
Have you checked for engine error codes?

MrGorsky · May 7, 2020

I'll do a wet test now. I have no error codes. It runs fine at tickover, but driving along at say 60 or 70 you can feel it misfire and stutter. Only does it when warmed up.

I had a dodgy throttle position sensor that I thought was the problem, so I've changed that and also changed the CPS.

When you accelerate hard with the windows open you can hear a slight chuff sound rebounding of the walls you're driving past. I thought it was a blowing exhaust or possible leaking exhaust gasket, but I've taken it all apart and can't see any evidence of a leak. I thought I'd see soot or something like that.

kermit_rr · May 7, 2020

Exhaust manifolds can chuff, so can loose plugs. I'm not sure what else that you've not checked.
I had backfires down the intake in hard acceleration, but that can't be confused with chuffing

MrGorsky · May 7, 2020

Wet test with WOT results are higher...

1=155. 2=175
3=175. 4=175
5=150. 6=200
7=160. 8=190

Henry_b · May 7, 2020

MrGorsky said:
Wet test with WOT results are higher...

1=155. 2=175
3=175. 4=175
5=150. 6=200
7=160. 8=190

Piston rings / bores worn..

6 and 8 are "around" the right place wet, but the rest are not brilliant..

Datatek · May 7, 2020

Henry_b said:
Piston rings / bores worn..

6 and 8 are "around" the right place wet, but the rest are not brilliant..

It's only done 158K miles

Grrrrrr · May 7, 2020

MrGorsky said:
Wet test with WOT results are higher...

1=155. 2=175
3=175. 4=175
5=150. 6=200
7=160. 8=190

#1 was the one with the suspect spark-plug?

Grrrrrr · May 7, 2020

Henry_b said:
Piston rings / bores worn..

6 and 8 are "around" the right place wet, but the rest are not brilliant..

Compression down on 1 and 5, ticking / chuffing but only when hot. Possibly coolant hose a rad hard? Could we be looking at a dropped liner here?

Henry_b · May 7, 2020

Grrrrrr said:
Compression down on 1 and 5, ticking / chuffing but only when hot. Possibly coolant hose a rad hard? Could we be looking at a dropped liner here?

Not likely, if they drop one it pukes it's guts out after every run, and you hear the "clack clack" noise associated with it..

knowing that the compression shot up when oil was added to the bores means rings/bores are worn..

DanClarke · May 7, 2020

+1 for exhaust manifold for the "Chuff". That was my problem on the Blue 4.6 Thor. New gaskets fixed it.

norseman · May 8, 2020

There's a worrying disparity between the readings.
Do you know the cars service history as 158K is not really excessive mileage for a RV8.

MrGorsky · May 8, 2020

I bought it 9 years ago with 67k on the clock. Quite a bit of service history came with it. I think it was laid up for a few years before I bought it because there's a hole in the service record.

I'm the 3rd owner from new, and I've serviced it by the book since then. Head gasket went at about 100k. That was Feb 2016. The bores looked great back then. You could still see the crosshatching from the honing.

It was laid up for a year in 2018 because I was out of the country.

Apart from the usual niggles it's been great really. I recently lost kickdown and it started misfiring and running rough when warm and at speed. Once it failed to start whilst hot. Kickdown was the TPS. That's been changed and kickdown is restored. Rough running is still there, and this funny chuff like a blown exhaust. Only does it when warmed up though.

It starts and idles very well.

If the engine is on it's way out, I think I'd like to try and replace it/repair it rather than throw in the towel.

Henry_b · May 8, 2020

Whats the blow by situation like?

If compression shoots up when oil is added, it is either bores or rings..

MrGorsky · May 8, 2020

Number 1 had a wet sooty plug when I ffirst took them out to have a look some days ago.

This afternoon whilst putting it back together a bit and changing the plugs, I noticed the connector in
Number 1 HT lead was pushed well back inside the boot although the plug was no longer sooty.

I've corrected that now. Hopefully that was causing the misfire.!!

Henry_b · May 8, 2020

It is entirely possible

P38A Compression Test Results.

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