Compression Test 88 XJS

here’s a good write-up on doing this. I used this first time I had to change spark plugs/compression test.

https://www.brianschreurs.org/neptune.spacebears.com/cars/stories/vee.html

I believe your mechanic is in for an interesting day :slight_smile:

Hello Greg - thanks for posting that link for the write-up of removal, and replacement, steps of the items in the V12 “center valley” when one is going to change the spark plugs - granted, the spark plugs “can” be changed using swivel joints on the socket as per the manual, without the removal of the items (I have done it), but this write-up sure explains better access for everything.

Few recall what a monkey wrench is and even fewer have one. I do !!!

Did you check to make sure the plug leads were going to the correct plugs? A couple crossed would still give you power from the V12 but possibly cause a rough idle.

I did not…and that’s something I’ll check out. It wouldn’t start one day and I pushed down on all the leads on the distributor and it started.
Thanks!

Compression test Thursday. I"ll let you know what’s going on. Hope it’s good, as I want to keep this car.

jaggxjs

Compression test Thursday. I"ll let you know what’s going on. Hope it’s good, as I want to keep this car.

SO took it across town, left it, and in 30 minutes they called and said…“it’s way over our head” LOL
SO will try one more place that I have used before for other newer cars. Waiting for a price on test.
Stand by all.

Next Thursday, different ship. About same price, but this shop does them often. I just didn’t want to go there as in the past they were kind of rude. Can’t be picky now.

Good tech, lousy bed side manner. Or Nice folks poor tech. Which do I choose???

Carl

Let me guess…Good tech.
You are correct sir.

Last week I had to get 4 new tyres fitted before the MOT. I went to the usual place, friendly enough. Despite me warning them that there was a different front/rear wheel offset they screwed up one side of the car. Luckily the MOT caught it and swapped the wheels back.

So I’d go with good tech.

Finally got the Compression test today. She rattled off numbers so quickly didn’t get them all…but
range from 175 to 191…most in 185 to 191 range. I"ll have them all soon.
What’s your thoughts on those?
Thanks for help.

My numbers on a COLD engine were 180-195.
Numbers on a WARM engine (was 160F when tested) were 215-225.

If yours was a cold engine, then that’s good. Your range is only 5 psi lower than mine. There is a 15psi difference between lowest and highest like mine too. My engine had about 50,000 miles when I did the test. So if your engine is about 75,000 miles, then I think you’re right on the money?

Changing oil a lot, and IMO running some MMO once in a while, will keep those numbers high.

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My numbers on a COLD engine were 180-195.
Numbers on a WARM engine (was 160F when tested) were 215-225.

If yours was a cold engine, then that’s good. Your range is only 5 psi lower than mine. There is a 15psi difference between lowest and highest like mine too. My engine had about 50,000 miles when I did the test. So if your engine is about 75,000 miles, then I think you’re right on the money?

Changing oil a lot, and IMO running some MMO once in a while, will keep those numbers high.

gregma, thanks for the reply.
I do not have all the details yet as they still have the car. Warm or cold don’t know.
I was hoping for your input here about the numbers, as I cannot find a reference as to what is good or bad.
The car has 49,000 miles, but speedo is intermittent, so may have more.
Your assessment is valuable as I told them to put in the NGK plugs gaped correctly, IF compression was good. Looks like it is. GOOD! I do not have their report on things. Will call them today to get more info.
They also found plug wires cracked. That is strange as I had them replaced 3 years ago…but might be that shop really didn’t replace at all…anyway I told them to replace those also.
More later…oh what is MMO?

MMO = Marvel Miracle Oil.

Andy_Broadbent

jaggxjs

16m

MMO = Marvel Miracle Oil.

Thanks!

Wow! I always thought it was Magical Mystery Oil. If it works, it’s magic. If not, it’s a mystery.

I’m more than likely wrong Dave - it’s a USA product & I’m in England :black_flag:󠁧󠁢󠁥󠁮󠁧󠁿

AKA pretty much Snake oil ???

I had to look it up! Marvel Mystery Oil. I’ve never used it myself, but people like it.