Compression test. Advise required. 1978 xj12
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From: Delegate
Compression test. Advise required. 1978 xj12
Hi all,
My 'barn-find Double Six has been slumbering for years. The fuel tanks and system are so rusty they look like they came from the Titanic.
I have the Fuel injected V12 stripped down to the cylinder heads.
I have removed the plugs and intend to compression test the engine. When I briefly ran it after taking delivery of the car the engine started easily but was definitely running on substantially less than all 12 cylinders. I am hoping it is not a major problem. First step, determine if the engine has compression.
Of course there are disconnected wiring harnesses etc. in the engine bay.
I believe the best way to crank the engine to conduct the test would be to attach the battery terminals and activate the starter motor by powering up the black and white wire running from the starter relay near the radiator to the starter solenoid, rather than energise the entire system via the ignition key complete with ignition and injection computers and a miriad of disconnected and potentially exposed terminals.. Obviously the fuel pump is already disconnected.
Is this approach valid?
Is there any potential damage that can be done in trying this?
Are there any alternative or better ways to operate the starter when the engine is largely disconnected?
Are there any tips or special procedures I should be observing?
With thanks
Alan
My 'barn-find Double Six has been slumbering for years. The fuel tanks and system are so rusty they look like they came from the Titanic.
I have the Fuel injected V12 stripped down to the cylinder heads.
I have removed the plugs and intend to compression test the engine. When I briefly ran it after taking delivery of the car the engine started easily but was definitely running on substantially less than all 12 cylinders. I am hoping it is not a major problem. First step, determine if the engine has compression.
Of course there are disconnected wiring harnesses etc. in the engine bay.
I believe the best way to crank the engine to conduct the test would be to attach the battery terminals and activate the starter motor by powering up the black and white wire running from the starter relay near the radiator to the starter solenoid, rather than energise the entire system via the ignition key complete with ignition and injection computers and a miriad of disconnected and potentially exposed terminals.. Obviously the fuel pump is already disconnected.
Is this approach valid?
Is there any potential damage that can be done in trying this?
Are there any alternative or better ways to operate the starter when the engine is largely disconnected?
Are there any tips or special procedures I should be observing?
With thanks
Alan
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Joined: Mar 2008
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From: Pacific Northwest USA
I don't have any Ser II diagrams but white/black may junction into who-knows-what.
Does your starter relay have a heavier gauge white/red wire? if so, it'll go right to the starter solenoid. I would jump it to 12v to engage the starter. I there's s smaller gauge white/red, I'd suggest that you don't power it up fro the reason you suggest. It's probably a trigger wire to the ECU.
Cheers
DD
Does your starter relay have a heavier gauge white/red wire? if so, it'll go right to the starter solenoid. I would jump it to 12v to engage the starter. I there's s smaller gauge white/red, I'd suggest that you don't power it up fro the reason you suggest. It's probably a trigger wire to the ECU.
Cheers
DD
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From: Delegate
Thank you Doug for your interest.
You are quite correct. I misread the microscopic wiring diagram in the manual. The correct colour code for the wire that should energise the starter is indeed red and white.
kind regards
al
You are quite correct. I misread the microscopic wiring diagram in the manual. The correct colour code for the wire that should energise the starter is indeed red and white.
kind regards
al
Joined: Feb 2010
Posts: 28,811
Likes: 11,280
From: Adelaide Stralia
Alan,
Doug is spot on.
With the battery connected as normal, and 12v applied to that wire will activate the starter and rotate the engine.
I would only do 2 to 3 second bursts, as that starter will get HOT.
A dry Comp test, followed by a wet test is what I usually do.
Good luck.
Doug is spot on.
With the battery connected as normal, and 12v applied to that wire will activate the starter and rotate the engine.
I would only do 2 to 3 second bursts, as that starter will get HOT.
A dry Comp test, followed by a wet test is what I usually do.
Good luck.
Senior Member
Joined: Jan 2017
Posts: 282
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From: Kerkrade, South Limburg, The Netherlands
For how long has the engine been in vegetation mode before you first started it?
As you already started it, the damage can already be done without you knowing.
But, on the other hand, it ran, be it not very well...
Compression testing is done dry at first. If that gives you readings that are close to or within factory spec, then that's it. Test successfull.
Wet testing (with a little bit of oil in the cilinders) will give you an idea of the cause of a bad dry test result:
If the compression goes up in the wet test, you can 90% surely say that there are some rings sticking. If the compression doesn't go up, probably there are some valves that will need attention.....
As you already started it, the damage can already be done without you knowing.
But, on the other hand, it ran, be it not very well...
Compression testing is done dry at first. If that gives you readings that are close to or within factory spec, then that's it. Test successfull.
Wet testing (with a little bit of oil in the cilinders) will give you an idea of the cause of a bad dry test result:
If the compression goes up in the wet test, you can 90% surely say that there are some rings sticking. If the compression doesn't go up, probably there are some valves that will need attention.....
Last edited by Dutch-Cat; May 6, 2018 at 04:39 PM. Reason: typo
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