I was given these numbers for my compression and leakdown test awhile ago... So I needed to rebuild or build a motor, but when I tell these numbers to people, they don't believe it would run well. The truck runs well and I fixed an a/f problem I had. When I pulled the plugs, they too didn't look bad... Are these numbers possible with a truck that appears to be running well.

#1 170 comp 5% leakdown
#2 175 comp 2% leakdown
#3 140 comp 25% leakdown
#4 110 comp 55% leakdown
#5 170 comp 60% leakdown
#6 165 comp 5% leakdown
#7 145 comp 13% leakdown
#8 155 comp 10% leakdown

The guy doing the work is great and wouldn't try to screw me over, but you know when you get to putting $7,200 into something that seems to run alright, you get a little scared... especially if you're broke like me.

 

Are these numbers after or before the rebuild?

Anything over a 6% leakdown, we find unacceptable.

Your motor may run, but does it run great? Just knowing that a motor needs to run in sync and not have such large tolerances should tell you something about the wide range of numbers you are getting is bad.

 

I haven't built the motor yet, just doing some feeling around... I would have thought with these numbers, my truck would run badly... It seems to run fairly well... I haven't noticed any oil loss or smoking... Now every once and awhile up near 4,200 rpm WOT, it will drop back 100 rpms and then go back forward with out any problems.

 

Maybe there is some fault in the tests? may not be accurate? My father is a self proclaim engine expert and his answer to everything is throw some Slick 50 in it and it will be fine. Maybe give it a shot, nothing to loss but 10 bucks.

 

ttt...

 

I would get your compression rechecked - the reason I say this is how can cyl #5 have a 170 psi comp with a 60% leak down?

A leak down can be caused through the valves, rings, gaskets, etc, but with that much of a leak down how can the compression be good? Leak down is kind of tricky to find exact tdc maybe a valve was open a bit durring that leak down?

BTW. why did you have a comp test done?

Did cyl #4 plug have any oil or anything? Was a wet compression test done on that hole?

You could always go to ford and get a relative compression test and a power balance test performed using the wds (should only be around 1 hr labour) That way if they come back with a low relative on cyl #4 and a misfire on #4 you'd have a better idea if that cyl or another one is bad. (cyl#4 is ofcourse based on your post and would be the worst of them all if the comp test is accurate)

 

exactly what i was thinking.

 

this thread will explain my problem



Thank god I went to Fast Lane Motorsports today. I went there because I had some free time and I hadn't had my truck on the dyno since I got it 3 weeks ago. Well we do a pull while the truck was still a little hot and got some good numbers until we took a peek at the a/f. JESUS!!!!! This truck was supposed to have been dyno tuned with a predator at JLP. Since the dyno tune was done, the guy I bought the truck from took off the 90mm MAF and cold air kit and left everything else the same. Well I put back on a 90mm MAF and bought a JDM cold air kit figuring that would have to be close.... Well long story short...

3,000-4,100 RPMS 16:1
4,100-4,400 RPMS 15:1
4,400-4,800 RPMS 14:1
4,800-5,400 RPMS 13:1


WTF!!!!

We then started a cold run after the truck cooled down and I was making more power but we had to cut it off at 3,500 rpms cause we didn't like the a/f numbers again.

I then figured the original program was in the computer and hooked up the predator and it showed a custom tune. Well I tried to call JLP and guess what, busy or no answer all day (how do these guys stay in business when I haven't heard of anyone being able to get a hold of them). So I called up Diablo and the lady who answers the phone was on lunch, so I was able to talk directly to a tech guy without waiting. I talked to Cole for awhile and I hooked the predator to my computer and emailed him my program and my graph. He told me for $120 they will work with me until my program is right... That is actually a good deal. I have not been abusive too bad on the truck since I got, but I can't believe I missed the spark knock. I am assuming when I had the 80 mm MAF on there I was fine, so I have run the 90 mm for about 500 miles with some WOT in there. What are the odds I scorched a piston or something like that?

That post was made on 8/08/2004

Because of those numbers I thought a compression and leakdown would be a good idea.

 

My 96 Cobra had 160-170 in 6 cylinders and 90-110 n 2 others. The car ran great other than the amount of smoke at WOT. I even put the car on the dyno with 2 bad cylinders and managed 300hp/300tq with a kenne bell. When the car had all 8 cylinders working I had about 380 hp.

The only reason I found out that I had a problem was the check engine light. The computer was detecting a miss fire in the 2 cylinders that had low compression.

So yes the truck could still feel fine with bad compression, the power will be down, but it could still run fine.

 

His numbers can look like that and I'm not suprised they do.

A leak down tester is metered....they don't flow alot of air so it doesn't take much of a leak to see a lot of leakage.

I had a #4 cylinder earlier this year show 125lb of hot cranking compression and when I checked it later with a leak test cold it showed 100 % leakage. This was caused when I burned the tip off the spark plug, a piece of the plug got stuck between the exhaust valve and seat leaving a ding in the sealing face of the exhaust valve. It took .015 to grind the ding out.

One thing you have to keep in mind is that a leak test is static...the engine isn't moving. It has all the time in the world to leak. A compression test gives less time for the cylinder to leak because everything is moving. Now when the enigine is running it has even less time to leak.....see where this is going.

I have heard of engines that showed horendous leakage and after the rebuild showed almost no power gain. But that all depends on were the leakage is.
Dale

 

so bad, with my numbers would you tear into it?

 

Here is what one of our more experienced engine builders said and goes along with what some of the others mentioned already.

"He can't be doing one or other or even both of the tests right.
You can't get high comp results AND high leakdown results on the same cylinder. Even the ones that have high leaks and lowish comp results, don't have a low enough comp figure to match the high leakage._All tests should be done at the same temp and both hot and cold tests_to help diagnose the problems. On the leak tests did he determine where the leakage was going (valves, bores, headgasket)? If not he wasted the best opportunity to diagnose his engine. A correctly carried out leak test is far more beneficial (and accurate) than a comp test or even a visual inspection (by most people). Ask him what his test procedures were, including time between tests, engine temp, etc."

 

on the 60% leakdown, it came out the exaust, you could hear it... I was told you can sometimes have i high compression and high leakdown... I don't know if that is true or not though... I was told it was making the compression, but it was unable to maintain it.

 

Well I got GREAT NEWS..... We hooked a vacuum guage up and it was very steady, we then did a compression test and I had 175 lbs on all cyl.... So I don't have to spend the money on a new block.... More bolt on's to come...