Compression testing question

[mdbrandy]

Ok, time to tune up the 280Z, and I wanted to test compression while I have the plugs out. Haven't run a compression check on any car for probably 20 years, so I opened up the factory manual to read what it wants me to do.

I assumed I had to keep fuel from being injected through the EFI system. Factory manual wants me to disconnect all the injector connectors and the cold start injector connector. AFTER running the engine up to temp. However, those connectors are pretty brittle by now, and I'll have to be very careful with them. It's likely to take 15 minutes or more to carefully disconnect all the connectors and remove the plugs. By then, the engine will still be warm, but by no means at operating temp. So how important is having the engine up to operating temperature in testing compression? Any other tips or tricks for measureing compression on an EFI engine?

[MikeW]

Originally posted by mdbrandy

I assumed I had to keep fuel from being injected through the EFI system. Factory manual wants me to disconnect all the injector connectors and the cold start injector connector. AFTER running the engine up to temp. However, those connectors are pretty brittle by now, and I'll have to be very careful with them. It's likely to take 15 minutes or more to carefully disconnect all the connectors and remove the plugs.

Can you just pull the fuse that feeds the EFI system?

[mdbrandy]

Originally posted by MikeW

Can you just pull the fuse that feeds the EFI system?

Interesting thought. Not sure whether there's one fuse or multiple, or whether I'd have to pull a fusible link. Also, the EFI is so integrated with the starting/ignition system, that I'll have to see if it will turn over if I do that. But it's a good idea. We shall see!

[ricksaia]

Warm engine, pull coil wire, push accelerator, turn over engine three or four times, get readings accordingly. No problem.

[sblake01]

Originally posted by ricksaia

Warm engine, pull coil wire, push accelerator, turn over engine three or four times, get readings accordingly. No problem.

If you do that, you'd still be pumping fuel. I'd just disconnect the fi relay and coil wire.

[78 280zcar]

Why dont you just do the procedure for changing the fuel filter in the manual. disconnect the - terminal from the battery, disconnect the black earth lead from the fuel pump, disconnect the the lead from the "S" terminal on the starter which would be the small wire. Remove the two screws which hold the cold start valve on and lift out the cold start valve and hold it in a container to catch the fuel and then reconnect the battery terminal and turn the ignition switch to start and all the fuel will be ejected from the valve thus relieving pressure and then you can do the tests and also change the filter. Shane

[mdbrandy]

Originally posted by sblake01

If you do that, you'd still be pumping fuel. I'd just disconnect the fi relay and coil wire.

Would disconnecting the FI relay disable injector pulsing but allow the engine to crank? That'd be good. Probably just take a minute to disconnect the relay.

ricksaia: Need to keep from pumping fuel during the compression test. Lots of raw fuel through the exhaust will supposedly screw up the cat converter.

78 280zcar: That'd work too, but would probably take as long as removing the injector connectors.

One question that no one has answered is how important is it to have the engine up to operating temp? Obviously the clearences are more correct at temperature, but how different would the readings be with a cold engine? Or with an engine that has cooled down for 15 minutes? I read about many people measuring their compression - do you make sure the engine is hot?

Thanks.

[Bambikiller240]

If the engine has been fully warmed, I don't think that a 15 minute delay is going to amount to much. The lack of coolant "flowing" should (I would thik) let the internal temps remain relatively "soaked" throughout the internals.

In doing compression tests on my 240Z's (L24) over the years, I've just warmed the engine thouroughly, pulled off the fuel lines to the carbs, pulled all spark plugs and coil wire, blocked the carb butterflies open, connected my remote starter, then tested. It had to take close to 15 minutes for me to do all that, as I don't work terribly fast. Compression has always been seemingly accurate (160-170psi, wet) on engines that were in good condition.

[mdbrandy]

Originally posted by Bambikiller240

If the engine has been fully warmed, I don't think that a 15 minute delay is going to amount to much. The lack of coolant "flowing" should (I would thik) let the internal temps remain relatively "soaked" throughout the internals.

In doing compression tests on my 240Z's (L24) over the years, I've just warmed the engine thouroughly, pulled off the fuel lines to the carbs, pulled all spark plugs, blocked the carb butterflies open, connected my remote starter, then tested. It had to take close to 15 minutes for me to do all that, as I don't work terribly fast. Compression has always been seemingly accurate (160-170psi, wet) on engines that were in good condition.

Thanks, Carl. Sounds like the quickest thing for me would be to warm up, disconnect the FI relay, pull the plugs, and go to it.

[sblake01]

The method I described works. You don't have to worry about the cold start valve as outlined by 78 280Zcar because it won't release the fuel in it if the engine is warm. Don't forget to pull the coil wire.

[mdbrandy]

Originally posted by sblake01

The method I described works. You don't have to worry about the cold start valve as outlined by 78 280Zcar because it won't release the fuel in it if the engine is warm. Don't forget to pull the coil wire.

Thanks Stephen, I did forget to mention the coil wire. Question, however - if I've removed all the spark plugs, and thus disconnected all the spark plug wires from them, what difference does removing the coil wire make? I can see it to keep any of the hanging plug wires from arcing to something that they shouldn't, but otherwise...

[sblake01]

Removing the coil wire will keep the spark from unnecessarily going the the distributor and eliminate the need to worry about the spark arcing from the plug wires.