P38A V8 Engine Condition.

Fellow forum members, I'd appreciate your thoughts on the following:

My car is a late 4.0litre Thor engined V8 fitted with a Prins VSI gas set up and is used as my daily driver. Anyway it's due a gas service soon and from the sooty deposits on the exhaust pipes I thought that it was probably running a little rich so I'd ask the technician to trim the fuel. Before I took it in I thought that I'd just pull the spark plugs to see that the condition was good and at the same time do a compression check on the cylinders.

The plugs all looked fine and whilst the pressure on most of the cylinders was right up there, one of them seemed a bit low; the pressures were as follows, Cyl 1 - 12.00 bar, Cyl 2 - 12.75 bar, Cyl 3 - 12.00 bar, Cyl 4 - 12.75 bar, Cyl 5 - 11.75 bar, Cyl 6 - 12.75 bar, Cyl 7 - 10.5 bar, Cyl 8 - 12.75 bar. If I squirt a little oil in cylinder 7 and crank it again the pressure will go up to 11.5 bar.

What do you think, is it okay or should I be concerned?

If i am correct 5&7 are right next to each other so would suspect the head gasket between the cylinders.

+1 on head gasket starting to degrade. While doing the compressions, were all plugs out, how long did you crank it for, and was the throttle completely open ?

FYI to make it easier, I have a toggle switch cable, that plugs in place of the starter relay. Then with fuel relay unplugged, I can hold the throttle open and run the starter for as long as needed.

pwood999 said:

+1 on head gasket starting to degrade. While doing the compressions, were all plugs out, how long did you crank it for, and was the throttle completely open ?

FYI to make it easier, I have a toggle switch cable, that plugs in place of the starter relay. Then with fuel relay unplugged, I can hold the throttle open and run the starter for as long as needed.

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I probably did it incorrectly then. I took out one or two plugs at a time and pulled fuse 28 (30A) . I ran the starter for about 5 seconds which I think turned the engine through about 4 cycles, I didn't hold the throttle fully open either. I did wonder if I should pull something else to stop the fuel pump. Do you think that I should repeat the test doing it as you suggest?

Yes try again. Few more turns for each cylinder with throttle open and note readings in PSI. it’s easier to compare.

all spark plugs out allows starter to spin it faster.

We're 5 and 7 tested last, dont forget the battery can drop a touch also.
I'd maybe retest to be sure to be sure!!

pwood999 said:

Yes try again. Few more turns for each cylinder with throttle open and note readings in PSI. it’s easier to compare.

all spark plugs out allows starter to spin it faster.

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Mark Piercy said:

We're 5 and 7 tested last, dont forget the battery can drop a touch also.
I'd maybe retest to be sure to be sure!!

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They weren't Mark. I started at 8 and went along that bank and then to 7 before finishing at 1. But I am going to retest over the weekend and do it according to sop.

Pul the fuel pump relay and the ignition relay.

Okay I’ve done the test again properly this time, thanks for the pointers Marshall, these are the numbers: cylinder 1 - 200psi, cyl 2 -190 psi, cylinder 3 - 200 psi, cyl 4 - 200 psi, cyl 5 - 180 psi, cyl 6 - 200 psi, cyl 7 - 180 psi, cyl 8 - 200 psi. Any thoughts people?

They should all be +/- 10%, which these are so all within spec I’d say.

Marshall8hp said:

They should all be +/- 10%, which these are so all within spec I’d say.

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+1. That'll do!

swac said:

Okay I’ve done the test again properly this time, thanks for the pointers Marshall, these are the numbers: cylinder 1 - 200psi, cyl 2 -190 psi, cylinder 3 - 200 psi, cyl 4 - 200 psi, cyl 5 - 180 psi, cyl 6 - 200 psi, cyl 7 - 180 psi, cyl 8 - 200 psi. Any thoughts people?

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For an old vehicle I would say that is a good result.

swac said:

Okay I’ve done the test again properly this time, thanks for the pointers Marshall, these are the numbers: cylinder 1 - 200psi, cyl 2 -190 psi, cylinder 3 - 200 psi, cyl 4 - 200 psi, cyl 5 - 180 psi, cyl 6 - 200 psi, cyl 7 - 180 psi, cyl 8 - 200 psi. Any thoughts people?

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That sounds fine especially for how old the RR is

Best thing now would be new spark plugs & air filter, plus check both O2 sensors are working correctly. Any OBD should work for engine if you don't have Nanocom.

Then go for a blast & get the LPG check. They should also be able to check the sensors on both petrol & LPG.

pwood999 said:

Best thing now would be new spark plugs & air filter, plus check both O2 sensors are working correctly. Any OBD should work for engine if you don't have Nanocom.

Then go for a blast & get the LPG check. They should also be able to check the sensors on both petrol & LPG.

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The car had a K&N air filter fitted about 8000 miles ago, at the same time that the plugs were replaced, the gaps all looked good when I took them out at the weekend. Both O2 sensors were replaced about 7000 miles ago because when it was last put on a computer it was working open loop instead of closed loop which apparently suggested both O2 sensors were bad so the car was working on a default fuel setting.

And thinking about it now I can’t remember if the Prins fuel settings were adjusted after the new 02 sensors were fitted. This may be the reason for the soot on my exhaust pipes? I think I’ve read somewhere that a little rich doesn’t hurt anyway, and preferable to lean any day.

It was definitely connected back up to a computer after the O2 sensors were fitted because the technician had to check that it was working closed loop but I’m almost sure that it was only checked on petrol. I’d better call the lpg man.

swac said:

The car had a K&N air filter fitted about 8000 miles ago, at the same time that the plugs were replaced, the gaps all looked good when I took them out at the weekend. Both O2 sensors were replaced about 7000 miles ago because when it was last put on a computer it was working open loop instead of closed loop which apparently suggested both O2 sensors were bad so the car was working on a default fuel setting.

And thinking about it now I can’t remember if the Prins fuel settings were adjusted after the new 02 sensors were fitted. This may be the reason for the soot on my exhaust pipes? I think I’ve read somewhere that a little rich doesn’t hurt anyway, and preferable to lean any day.

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Soot in exhaust quickly poisons the cats and the sensors. New sensors may well require trims resetting but a long motorway ride should do it. O2 sensors can be tricky so work checking values fluctuate once engine is warm.