4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

4.0 ohv compression ?s

  #1  
Old 05-15-2019
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4.0 ohv compression ?s

I have a 98 ranger 4.0 ohv 5 speed manual. I have been tracing a bad bucking/misfire for months. I have replaced almost all fuel and spark parts and Iím sure they are all working. So I looked to compression .I got my hands on a compression tester tonight. I had 150psi on 5 cylinders and 130psi on one. My question is that enough of a loss to cause the whole truck to lose power? It literally feels like the truck is running on 3 cylinders. I have to ride the clutch to get it moving. Thanks
 
  #2  
Old 05-15-2019
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Yes, compression is power, so low compression will make for low power

150psi is about right, 130 is low, more than 10% but would only cause random misfires

Compression is where you should start, because it fairly black and white, and not intermittent

Then check firing order on coil pack and each spark plug
Coil pack, these are the Matched Pairs, 3/4, 2/6, 1/5
3 4
2 6
1 5

The 5 6 4 side always gets me,very very easy to reverse 2 wires at the coil or the spark plugs
Check it 3 times, then have a friend check it, lol
Don't even want to think of all the HOURS I have wasted, chasing my tail when I had reversed spark plug wires, that I would SWEAR were correct
Misfires from wrong firing order really limit power

A cylinder needs 3 things to Fire
1. Spark, at the right time
2. fuel, in the correct mix with air
3. compression above 65psi<<<< thats running compression, which is about 1/2 of static compression, i.e. compression test is 150psi then running compression is 75psi

Your compression is OK
That leaves 1 and 2
Check spark plug wires, easy to do, also spark plugs on this engine need a big gap 0.054", smaller gap means misfires, too large as well

Fuel is not easy to test, because it is a mix with air, computer does "on the fly" calculations
First thing about FUEL is don't trust it, lol, I have also spent more hours than I care to admit on "Bad Gas", gasoline with too much water in it, low power and misfire, the gas looks OK, smells OK, but is NOT OK
You said you been battling this for awhile so we will assume gasoline is good, but it is an ASSUMPTION

The computer uses MAF sensor, and O2 sensors to calculate air fuel mix but ONLY after engine is warm, say after 2 or 3 minutes from cold start
On cold start computer ignores MAF for the most part, and ignores O2s completely

The computer is programmed at the factory for this 4 LITER engine, so it "knows" how many LITERS of air are being sucked in at any RPM and any throttle position, its JUST MATH
Computer then adds fuel based on that air flow.
In 1998 Computer expects 55psi fuel pressure at the injectors, so it bases open time of an injector on that, if fuel pressure is only 20psi then not enough fuel flows in
That should set a LEAN Code

Do you have any CODES now or have you had any Codes in the past?
Does the Check Engine Light(CEL) work?
Should come on with the key

What the computer doesn't know is the WEIGHT of the air, it varies by elevation and temperature, and the WEIGHT is important because the air:fuel ratio for gasoline is 14.7:1 and that's a WEIGHT RATIO
14.7 pounds of air gets 1 pound of gasoline mixed in
14.7 grams of air gets 1 gram of gasoline mixed in

So unplug the MAF sensor and see if engine operation changes
Thats the point of the MAF sensor, it measures the WEIGHT of the incoming air
And there also an air temp sensor to back that up.
MAF also only samples 10% of the air, if if its off just a little thats a big change in weight

Also unplug the Cam sensor, at a different time, to see if you have better running

These sensors rarely fail but are often replaced

ECT sensor can be replaced as they do get wonky, they usually cause Rich running, or bogging after full warm up
 
  #3  
Old 4 Weeks Ago
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Compression

Yes Iíve had check engine codes. They jump back and forth between bank one lean and bank two lean and bank one rich and bank two Rich. I have tried unplugging and cleaning MAF. It misses as soon as I start the vehicle. I have replaced all three O2 sensors and the fuel injectors. I have replaced the fuel filter and injectors. Tested fuel pressure on the rail itís right about 60. I have also run the truck completely out of fuel since the issue so Iím pretty confident itís not bad gas. I will check plug wires when I get home later today and cam sensor. Would be nice if it was a plug wire missmatch.
 
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Old 4 Weeks Ago
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Compression

I have also replaced coil pack plugs and wires. And it started doing it before I replaced plug wires. So Iím semi confident itís not mismatched wires. But I will double check.
 
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Old 4 Weeks Ago
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Take off the Computers plug, on upper firewall, and have a look at the connections, corrosion
Both Lean and rich means there is a computer issue, because of connectors location it can get water inside, hopefully thats the problem, otherwise you may need to new computer

When you ran without the MAF did you get a MAF code?

Also try doing same thing with CAM sensor

You are testing computers "sanity" as well as the sensors
 
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