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Compression?


HMFM Miller

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Any one know the cylinder compression for the 3.0? I have an 2003 ranger and Im chasing a misfire on one cylinder.
 


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Ford doesn't generally issue compression specs in terms of PSI.

90 is the minimum needed to run, and it will barely run with that. 130 is good, 150 is better, more than a 10% drop from the highest number to the lowest number is unacceptable and will cause a misfire on the low cylinder.
 

HMFM Miller

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Im gettin about 115 on the cylinder in question. Havent checked the others. Its making me think I got some kinda valve issue going on.
 

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Check the other cylinders.

Unless you fall below that magic number of 90 a single cylinder's compression number isn't useful. They can't stand alone because it is hard to predict how an engine will wear, and also because the what you feel as a misfire is just the stutter of one cylinder contributing less than the others.
 

HMFM Miller

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Thanks, Ill check tomorrow.
 

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My hot engine numbers are 185-195. Done it twice since new, hasn't changed.

FSM states everything OK if maximum pressure does not differ by 75% of minimum pressure - which is a huge tolerance. Leak down test should be below 20%.

It may be an indication of Ford's wide manufacturing margins (read poor quality control) that it won't give a specific psi. My Camry and Mitsubishi both do.
 

HMFM Miller

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Hahaha "wide manufacturing margins".
 

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It may be an indication of Ford's wide manufacturing margins (read poor quality control) that it won't give a specific psi. My Camry and Mitsubishi both do.
Ford used to give PSI specs, but then people started trying to diag off of them, to ill effect. Frankly, for misfire diagnosis a percentage change is more useful since the misfire is actually just a low spot in the power cycle. If you would lower all other cylinder contributions to match the "bad" one, it wouldn't be a misfire (assuming the engine still ran).
 

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Any one know the cylinder compression for the 3.0? I have an 2003 ranger and Im chasing a misfire on one cylinder.
There is a TSB about valve seats coming loose on the 2002-2006 3.0ls

Cause intermittent misfires

Engine compression has so many variables in the testing methods that to give a specific range is next to impossible.
All spark plugs need to be removed
Larger vacuum line should be removed or throttle needs to be propped open to let air in to be compressed.

Then you have crankshaft speed, which is the point of removing all the spark plugs.
If battery is low then crankshaft speed will be less, new battery faster crankshaft speed.
Metal rings against metal cylinder walls and metal valves against metal valve seats don't provide a long term seal, they leak and should because of the metal to metal "seal".
So the faster the piston comes up the less time there is for air to leak out so the higher compression number you will see.

Some like a warm engine compression test because heated metal expands so a better "seal" will often be seen.
For what a compression test tells you I don't think that is needed, but to each his own.

With above parameters used, I would expect a 9.1:1 to 9.3:1 engine to show approx. 175psi with a 15% deviation at most between cylinders.

A compression test is not for 1 cylinder, info is useless without a baseline, so all cylinders need to be tested and then compared.
 
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HMFM Miller

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Thanks for the info. I got called in to work today and didnt get a chance to mess with the truck. My 89 silverado is my DD till I get the ranger back up to running.

I picked up a vacuum gauge so Im hoping I can get a better idea on what the trucks doing.
 

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A vacuum gauge is one of the better diagnostic tools to use for the mechanical condition of an engine.
 

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Ford used to give PSI specs, but then people started trying to diag off of them, to ill effect.
The factory psi spec should be meant to mostly judge an engine's wear, if all cylinders are about the same. It can be useful to help diagnose fuel use and low power. Not knowing the design psi makes the buying process of used vehicles less transparent and reduces a manufacturer's liability. The only better judge of overall wear would be HP output and used oil analysis.

My Camry lists the new compression spec as 175, it now has 200K with recent compression at 170 in all cylinders.
 

HMFM Miller

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So i hooked up my vacuum gauge. Its reading a steady 15in at idle. It drops when you crack the throttle open and rebound like I believe its supposed to.

Is that a low enough reading to cause my misfire?

I pulled the pcv valve while it was running and the vacuum didnt change. Does this mean the pcv is bad?
 

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Google: engine vacuum test results

15 is a little low, are you at sea level or in the mountains?
So you could have a vacuum leak.

PCV valve is closed at idle, so did you just pull it out of the valve cover or did you remove the valve from the hose?

If you shake a PCV valve you will hear/feel a little ball/plunger inside, that "ball" will be sucked up by the high vacuum at idle to close the hose to the intake , then as throttle is opened vacuum drops and so does the "ball", at that time oil vapor is sucked from the valve cover/crankcase.

To chase a vacuum leak, with engine idling, pull off each vacuum hose(one at a time) on the intake and plug the hole in intake with your finger, watch for higher vacuum than 15.
 
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HMFM Miller

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Im at about 1600 feet so it should drop a couple in but not that much I think but Im not sure. I pulled the pcv from the valve cover. At idle, I put my finger on the port of it and the pressure went up an inch or so.

You guys are awesome.
 

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