compression testing a coyote

[skaarlaj]

The two concern are the drive by wire throttle body, how to go wot to let air into the motor to compress while cranking, surely the computer wouldn't allow any throttle while cranking. I was considering removing the throttle body, that's about as wot as you can get.

My other concern is, will the motor still keep on cranking because it won't start, as all that going to the "start" position with the key really does is start a procedure allowing the ecu to start the engine it's own way. I guess I can just be ready to disconnect the battery.

Finally the reasoning. The car just seems to not be performing as it should. My G-tech shows it barely trapping 100mph in the quarter, when my manual car that was stock except for an axle back trapped up to 112 a couple of times with the stock gears, and regularly trapped 107-108 with the 3.90s.

Also, right now I'm in the process of tuning, and the ecu for some reason isn't wanting to lock the converter even at cruise, but I'm confident we'll figure that out. But I also smell a little bit of burnt oil from time to time, but have never seen an ounce of oil smoke out of the tail-pipes, and haven't seen a drip on the driveway or under the hood, but the smell is there, so I want to just make sure all the cylinders are at least equal to each other with cranking compression.

 

[skaarlaj]

Well I got it done, just removed the throttle body and set it aside, and after about 10 compression strokes per cylinder tested I just turned the key back to "off" and it worked fine, here's my results.

#1=129psi
#2=128psi
#3=130psi
#4=128psi
#5=128psi
#6=131psi
#7=131psi
#8=120psi

I know the number 8 is a little low about 10 psi ish, when compared to the other cylinders, but I doubt it's anything to be concerned over. Thoughts?

 

[weather man]

Less than 10% difference, let her buck.

 

[BruceH]

Well I got it done, just removed the throttle body and set it aside, and after about 10 compression strokes per cylinder tested I just turned the key back to "off" and it worked fine, here's my results.

#1=129psi
#2=128psi
#3=130psi
#4=128psi
#5=128psi
#6=131psi
#7=131psi
#8=120psi

I know the number 8 is a little low about 10 psi ish, when compared to the other cylinders, but I doubt it's anything to be concerned over. Thoughts?

Have you been able to find a reference? IIRC they should be closer to 160-180psi but I can't find a spec on it.

 

[skaarlaj]

Have you been able to find a reference? IIRC they should be closer to 160-180psi but I can't find a spec on it.

I think it may be my elevation here at 6500ish'. I'd love to have another Coyote to test side by side to check against, but I recall my old 91 5.0 fox with between 125-139 psi, and it seemed to run pretty strong.

Also with only 32,000 miles showing, I'd guess there would be just a problematic cylinder or two with such low miles, and not the whole motor down, but I could be wrong. Here's a little article that sorta made me feel like my engine's sealed up pretty well.http://www.gregsengine.com/cylinder-compression-testing.html

 

[ford20]

Is this what you are referring to Bruce?

Have you been able to find a reference? IIRC they should be closer to 160-180psi but I can't find a spec on it.

 

[Thereaper]

120 on #8

 

[weather man]

120 on #8

He's at 6,500 asl.

 

[weather man]

Tidbit I found.

Altitude and temperature also affect the compression readings. Manufacturer’s specifications are almost always given at a specific altitude (14.7 psi at sea level), and 59° Fahrenheit. Both temperature and barometric pressure change as you go up in altitude, so you will need to correct your measurements if you wish to compare it with a factory specification. The following chart provides conversion factors for correctly compensating for changes in altitude:

Compression Test Altitude Compensation Factors
Altitude Factor
500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773

A standard compression reading of about 150 psi at sea level in Los Angeles would measure significantly less in the surrounding mountains. For example, at an elevation of 6000 feet, the expected reading would be 150 psi X .8359 = 125 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude.

 

[skaarlaj]

120 on #8

It's 120, not 35, are you serious?? 120 of the best at 131, is an 8.4% loss, and only a 6.25% loss when compared to the 128psi cylinders. I'm betting there's greater variation between cylinders on a ton of these cars, but they haven't been tested is the only difference. But I am all ears, hahaha.

 

[skaarlaj]

Less than 10% difference, let her buck.

I'm thinkin the same thing as you. I was actually pretty pleased not to even see a 10% variance like I've seen on a fair amount of great running motors.

 

[Marble]

We never removed the throttle body when we have tested mine. Doesn't seem necessary.

 

[skaarlaj]

We never removed the throttle body when we have tested mine. Doesn't seem necessary.

I know that on carbureted, or non drive by wire vehicles you open the throttle all the way while cranking. I don't know how much vacuum or if any at all can be created by the engine spinning at "starter" speeds, but if there was vacuum present during the cranking, you may have to crank quite a bit more to reach the same cylinder pressure, than if there was no inlet restriction while doing the procedure.

I'm sure with a drive by wire vehicle, you could floor the accelerator, and it wouldn't open the throttle at all, so My theory was to just remove the four bolts on the throttle body, and set it to the side to best mimic a non restricted "full throttle" condition while cranking like on the older vehicles that actually had throttle bodies mechanically connected to a gas pedal with a throttle cable.

Truth be told, I may have gotten the same results with the throttle body in place and closed?

 

[skaarlaj]

We never removed the throttle body when we have tested mine. Doesn't seem necessary.

What kind of pressures did you find when you tested if you don't mind sharing?