I feel like I should have found the answers to these questions on RL, but I have come up a bit short. I would like to run a compression test on my engine (S4), and one thread from 2003 (https://rennlist.com/forums/928-foru...n-testing.html) answers many of my questions, but I was still wondering (i) how do you disable the ignition system on an S4, and (ii) how do you hook up a remote starter to an S4 so I can control the test myself, (iii) why is the the throttle wide open?

I had a read through the WSM and parts catalog hoping to to try and get a feel for what needs to be done but no dice. These are areas I probably have the least automotive experience with so any help you can offer would be greatly appreciated.

Thanks,
Dave

 

Right off my head I think if you kill the fuel pump (remove fuse) it would't run and cause flooding or pull the high tension lead on off the coil (I prefer to kill the fuel pump). At the front battery post, there is a large wire that goes to the starter solenoid. If you are able to make a connection to that wire (if you don't want to crawl under the car) and the other to the battery post she will turn over when you press the button on the remote starter. Ensure first that the car is in neutral first

 

Use a jump starting trigger tool, connect it to the hot jump post, pull the 14 pin connector, and it's the front left pin.

 

You do not want to pull the HT lead on the coil to disable the ignition. When you do this you disrupt the ground path through the plug wires and spark plugs and create a ground path with essentially infinite resistance. This can cause coil damage as the secondary winding normally dissipates its voltage through the disconnected path. The secondary winding will now produce significantly higher voltage in an effort to find a new arc path to ground. This higher voltage can cause breakdown of the coil insulation when the energy is forced to dissipate with in the coil itself since there is no ground path. If you are going to disable the ignition by removing the coil wire the safe way to do this is to disconnect it at the distributor end and the ground the coil wire to provide a ground path for the spark energy to dissipate through.
A much better alternative would be to disable the ignition on the low tension side. This can be done numerous ways. For example you could pull the EZF relay, or you could disconnect the harness at the EZF itself. You could also disconnect the low tension wire at the coils the small wires bolted on opposite sides of the HT lead….

EDIT: the reason you hold the throttle wide open is that you want to eliminate the restriction in the intake system. if the cylinder can't fill with air because the throttle isn't allowing any to enter the engine the compression numbers will be artificially low.

 

Do it Sean's way and no need to worry about the ignition or fuel systems, since the ignition key is off. This method is good if you are careful making the connection to the 14-pin, and do not allow the clip from the remote-start switch to touch any other pins in that connector. Alternately, you can attach your remote switch directly to the trigger connection on the starter solenoid.

Earlier cars have a resistor that feeds the ignition circuit during cranking. If you are lucky enough to have one of those cars, you need to remove the col wire from the distributor and ground it securely during compression-test cranking. Failure to observe this risks damage to the coils and possibly the final stage(s) that feed power to the coil.

For folks who want to do compression testing using the key switch rather than a remote button as the OP is doing, the safest way to disable the ignition and injection on post-CIS cars is to remove the harness connectors from the ignition and FI brains in the passenger footwell area.

Side notes:

-- Take ALL the plugs out during testing.

-- Keep the battery charged during testing. Else the later readings at lower cranking speeds will be lower.

-- Keep the throttle propped open during testing. This ensures equal cylinder filling during cranking, for both accuracy and cnsistency of readings.

-- Write the readings down in your logbook. Five years from now when you are trying to figure out why the car isn't running well, the readings in the logbook will be the baseline for the next set of readings. If you don't keep a maintenence logbook with the car yet, this is a good time to start. It's as easy as a small notebook in the glovebox. Mine has a listing of every task done to the car, every dollar spent, with dates and odo readings. My log includes fuel useage too.

 

dr bob makes some excellent points, and I would like to add a couple of comments.

"Earlier cars have a resistor that feeds the ignition circuit during cranking. If you are lucky enough to have one of those cars, you need to remove the coil wire from the distributor and ground it securely during compression-test cranking. Failure to observe this risks damage to the coils and possibly the final stage(s) that feed power to the coil."

I don't think that dr bob emphasized enough that this must be done on the early cars WHETHER OR NOT THE IGNITION SWITCH IS ON when you run the test. When the starter is energized on the early cars, whether it is done with the ignition switch or with a remote starter hooked to the fourteen-pin connector or the starter solenoid, there is power applied to the ignition system. To repeat: On an early car, you should pull the high-voltage coil wire from the center of the distributor and safely and securely ground it to the engine.

If you don't kill the injection while doing the test, each time that you test a cylinder, you wash more of the oil off the remaining cylinders. By the time you get to the last cylinder, it is a dry test, and the reading will be incorrectly low.

I think that you need to pull the plug wires and loosen each plug a turn or two, then snug them back down. After doing all plugs, crank the engine and rev it a few times. This ensures that you won't have a bit of crud under an exhaust valve giving you a bad reading, and that you have oil on the cylinder walls.

Pull the plugs, and keep them in order, so that you can know which cylinder that oil-soaked plug came from after you examine them with a good light and a magnifying glass.

I taught a tune-up course for years, and one of the things that I taught was that the most important diagnostic tool that you have is your records. The compression readings are certainly an important part of your records, so I strongly agree with dr bob - write the readings down in a log book or your manual, or somewhere that you will keep.

 

+1
I do one every spring when I check / change the spark plugs.

 

Throttle wide open reduces the vacuum loss when the piston is on the down stroke, thus improving volumetric efficiency. When you do this in the future, repeat the test with the throttle open again.

IMO, leave the ignition alone, and disable the fuel delivery by removing the fuse from the fuel pump. Run the car for a few seconds to purge trapped fuel, and begin the test.

It wasn't mentioned but the test should be done with the engine fully warm.

 

I strongly disagree with the 'leave the ignition alone' recommendation. It must be disabled to avoid damage to the coils. The ignition system will generate voltages that exceed the insulation capabilities of the coil secondary windings unless a suitable path to ground is provided. That path is normallt through the plugs. This warning is especially applicable to cars with aging ignition coils. That internal arc path can as easily be to the hot sideof the primary circuit as to anywhere else, and that means a high-potential arc will go back through the transistor box on the front apron. Not good for those parts.

Grounding the coil wire is simple and effective. Why risk damage when the insurance is so cheap and easy?

 

Any idea when the "early" car with starter fed ignition ended?

 

Ok: When removing the plugs, just put them back on the coil wires and lay the plugs on something metal.

Happy?

 

I'm always happy. My compression is still OK even at my advanced age. Leakdown... That's a different story. So I drink a little less and manage the mess.

What was the question again...? Oh yeah. Ground the coil wire. It's easier.

 

TMI, you old fart.

 

In the USA, the ignition systems changed in 1985.

For the RoW cars, I suspect the answer is that if you have a distributor or distributors, you need to ground the coil wires under any circumstances. If you have caps on the timing belt covers, you need to either ground the coil wires - note, wires - kill the ignition in some way, or spin the engine without turning the ignition switch. Probably still easier to just ground the coil wires. Stay away from wire harnesses and fuel system parts.

 

Sean, thank you...talk about a picture being worth a 1000 words! I cant tell you how helpful that was!!!