I'm going to be taking a look at several used F150s this weekend. Mostly of the '87-90 vintage. Most have around 100k-130k miles.

First one is a 1990 4x4 with the 4.9L I-6. Have compression tester, but do not know what type of values are considered "acceptable". Anyone help-out here? I would like to know the accepted range and the max acceptable difference between cylinders.

Same deal with the 302 and 351's.

thanks,

Mark

 

The lowest reading should be within 20% of the highest reading (ie: 150 psi-120 psi).

If the test is performed with the engine at operating temp., all the plugs removed, and the throttle wide open, expect the minimum:

4.9L reading to be 125 psi or higher
5.0L reading to be 130 psi or higher
5.8L reading to be 135 psi or higher

Readings below these numbers, when tested as described, are, IMO, too low.

The most important factor is the range from high to low. 25% is often considered acceptable, but not by me. If I were buying it, and didn't want to rebuild the engine, I would not accept more than a 10% difference between high and low, but I'm picky. If the engine is not at operating temp. when the test is performed, the readings will be lower.

Again, these are just my preferences.

Take care,
-Chris

 

Why do you have to have the throttle open? I'm not too keen on that, with a F.I. motor (especially when it's not my motor). Also, what if the test is done on a cold motor?

 

Have you ever done a compression test before? The throttle has to be open so that the cylinders can get air. This is mandatory regardless of whether the engine is carbureted or injected. If you don't, the numbers are meaningless. The same is true with the engine temp. If the engine is cold, the numbers are meaningless, and completely inaccurate. If the engine is warm, the numbers will be close as far as the range between high and low, but will be significantly lower than if the engine was at operating temp. You have nothing to fear by setting the throttle at WOT for the test. In fact, that is how you prevent any fuel from being injected during the test (the ECM shuts off the injectors at WOT during cranking) in a fuel injected engine. Just remember to disconnect the primary wiring to the coil (for either a carburetted or injected engine).

If they will let you perform a compression test, surely they will let you take it for a test drive, right? Do the test drive first, then the compression test. This way the engiine is at operating temperature for the compression test.

Take care,
-Chris

 

CHris:

Thanks for the info. No, I have never done a comp test - only watched someone do it.

 

I'm sorry, I presumed that you had. Otherwise I would have explained things better.

Important things to remember:

1) Engine needs to be at operating temp.
2) Throttle must be at WOT for entire test.
3) Remove all of the spark plugs before testing, and don't reinstall any of them until the test is complete.
4) Disconnect the coil before testing.
5) Crank the engine for 5 complete revolutions for each cylinder when testing. Be sure to do this the same for each cylinder.
6) Write down each cylinder's results before proceeding to the next cylinder.

Also, take a pair of heavy gardening gloves with you. The exhaust will be hot, and they will help.

That's about it. If you have any questions, just ask!

Good luck!

Take care,
-Chris