Compression numbers

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May 16, 2009 | 06:47 PM

Poopy

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Compression numbers

I have searched and cannot seem to get a straight, definite answer on this. What is the factory compression psi for a 95 Ford 4.9 300 straight six? Also, how do I do a compression test on this particular vehicle? On most of the cars I do tests on, I can pull the MPI fuse or like so the car doesn't spark or pump fuel. Where is something like that I can pull on this truck(95 f150) so I can do a test without starting a fire. I really need to know because I am going to buy one tomorrow as long as the compression tests out. Only reason I am iffy is because it has a pretty rough idle. While driving it drives great and pulls great, but I am worried it might have a low cylinder. Just want to make sure before I buy. I'm sure this has been answered, but I can't seem to find it.

Thanks, Kyle

May 16, 2009 | 07:39 PM

phoneman91

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Joined: Jun 2002

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From: Aurora,Colorado

Quote:

Originally Posted by Poopy

My Ford Factory Service Manual says the lowest cylinder reading should be at least 75 percent of the highest cylinder reading. No official PSI minimums it seems. The compression ratio is 8.8 to one. I would think that anything over 120 PSI with cold engine would be good if all cylinders are close in PSI readings.

If you crank the engine with the accelerator pedal to the floor-WOT at cranking speed automatically turns off the injectors. Or you can remove the fuel pump relay and then crank it. I would prefer removing the relay. The FPR is located in the black relay box just in front of the driver's side hood hinge on my 92 F150.

May 17, 2009 | 06:09 AM

fmr9

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Don't know if my numbers are typical, but I did a compression test a while back on my 300. It has a little over 200,000 miles on it and I was just curious about the compression. All cylinders were between 178 and 182. I was a bit suprised that they were so close together.
I learned to do compression tests a long time ago, on carbed engines. The thought of the injectors firing and a possible fire never occured to me. I used my little antique remote starter hooked to the fender mounted solinoid. Don't think I had the ignition on, so not much chance of a spark, but pulling the fuel pump relay sounds like a good idea.
Good Luck Frank

May 17, 2009 | 07:55 PM

thegreatbriguy

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What about the inertia switch? It you trip it, then no fuel will be delivered to the cylinder. I think it's in the cab?

Brian
89 F-250

May 18, 2009 | 09:23 AM

alxsnmr

Fleet Mechanic

Joined: Mar 2005

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From: St Louis

The inertia switch will kill the power to the full pumps without having to pull the fuel pump relay switch. You can just unplug the coil from the power supply and not have a spark to the cylinders.

May 18, 2009 | 02:38 PM

phoneman91

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From: Aurora,Colorado

Quote:

Originally Posted by alxsnmr

The inertia switch will kill the power to the full pumps without having to pull the fuel pump relay switch. You can just unplug the coil from the power supply and not have a spark to the cylinders.

I didnt think that it was possible to trip the inertia switch open manually.

Just pull the fuel pump relay-it just plugs in and then crank the engine before removing the spark plugs-there will be no residual gas injected into the cylinders when testing the compression.

May 18, 2009 | 03:56 PM

alxsnmr

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From: St Louis

If it is like mine, you should have a slot just behind the button. A paper clip works well enough to push the button out and then the switch has been tripped. Push the button in and the switch has been reset.