Have a friend with a 2006 F150 that runs rough. Has the 5.4 with 150K miles.

He took it to a shop, they report that cylinders 2 and 4 have low compression. This is from the built-in computer, so they didn’t do a traditional compression check.

Other symptoms: It runs very rough at idle, dies, seems to run better at 20 to 30mph.

First, I’d like to ask: Is this reliable/correct? Does this mean these cylinders actually have low compression, or could it be something else?

Both of these cylinders are on the right/passenger side of the vehicle.

What do you think the problem could be?
Is is probably either valves or the cam shaft?

Is it possible to take off one head and check/repair valves and/or cam shaft?
Since the problem is only on one side?

The shop tried to say that he should just replace the motor, but that seems kinda expensive, he’s looking for a cheaper option, would prefer to fix it.

Thank you!
Larry

 

The PCM doesn't know what the compression is. A real compression test should be done on all cylinders of this engine. Also, get all codes stored in the PCM and post them here. That will help us help you help him.

 

Thanks for your response. The check engine light is NOT on, I plugged in my code reader and it says there are no codes. He drove it home from the shop, about 5 miles. It kept dying at stop signs and lights, ran better at higher RPMs, I don't know why it wouldn't throw a code. Although I think I have a low-end/basic reader, I assume the pros can do more diagnostics.

The shop showed me their program that showed 5% relative compression for cylinder 2 and 25% relative compression on cylinder 4. What does that mean? Are you saying that could be wrong, that the engine could have a different problem, not really have a bad valve or something?

 

I would first suspect a blown head gasket. An actual compression test of each cylinder should be done to confirm that.

 

I am going with broken lash adjusters and valves hit pistons.

 

The relative compression test on the scan tool is fairly accurate at pointing you in the right direction. The next step us to verify with an actual compression tester. Just pissin in the wind until that is done.

 

I agree with @hivoltj , and others here. This is another example of how DAMN fast the PCM computer is and what it can actually figure out when it has nothing else to do but monitor drive train operations - then how DUMB computers can be at other times.

The shop is talking specifically about a 'power balance' test. This is actually a function of the misfire detection routine in the PCM. That routine determines a misfire literally by detecting the _absence_ of the short term rise in crankshaft rotation speed that occurrs immediately after a spark event is commanded. There is a threshold (??? IDK what) where it logs it as a P030x code. You are slightly above that threshold on Cyl 2 & 4 where it is NOT setting a misfire DTC. But those cylinders are producing less crankshaft acceleration that the others (by the percentages you mention).

Amazing the computer is fast enough to detect THAT minute a difference - and as @hivoltj said it is quiet accurate. Power produced by those two are lower than the others. Compression low??? Maybe / Maybe NOT, but I would bet it is. WHY? Could be head gasket, or valve, or compression rings, -- or heaven forbid BENT RODs from fuel hydro-lock. All those would result in low compression. The computer can't figure out which of those it is, but your symptoms would be same in any case ((( Power produced by those two is low ))).

Real Compression test, or borescope, or remove valve cover is in order next.

 

Thank you very much for all the great suggestions and to Torqued for the more detailed explanation!

OK, I’ll see if we can get those spark plugs out.

Do you think it’s significant that they reported Cylinders 2 and 4 have low compression?
That 2 and 4 are NOT next to each other? Does that mean anything?
A mechanic that I’m working with thinks that it’s likely NOT a head gasket because those 2 are not next to each other and that if the head gasket went out, it would be between 2 cylinders next to each other. Is that correct/helpful?

Any thoughts on: What does it mean that specifically 2 and 4 have low compression?

Also: What would I look for if I removed the valve cover? Can you see if the valves are damaged by removing the valve cover, without removing the head? Can you tell me about that test?

Thank you!

 

You are correct about cylinders 2 and 4 not being next to each other and head gasket might be somewhat more likely to effect adjacent cylinders. Not absolutely depending on what event brought on the problem. Overheating can 'warp' a head causing gasket leak in any place or multiple places. I think- if that is the case you would have overheating problems.

There are a couple of other different reasons that can result in certain Cylinders having low compression. Cam / Roller / lifter problems (but would be associated with NOISE by the time if effects compression). Valves or Valve seat not sealing tightly (But these engines do not generally burn valves - especially on multiple cylinders). Piston Ring wear or cylinder scaring (from poor Air Filter Maintenance or from poor oiling - poor hygiene on plug jobs letting junk / carbon get into a cylinder or broken plug tip or broken ceramic getting into cylinder).

Pulling valve covers - you can SEE what is working or not working - but probably not what is causing low compression.

If in your shoes - I would do a Wet & Dry compression test on all cylinders. (Wet / Dry can give some indication of which [rings / valves]. (Make sure battery is fully charged. Remove all 8 plugs. Install compression guage - press accelerator to floor (de-choke) and run starter 3 - 5 seconds, and record reading. Remove gauge, spray about a five to six or seven second blast of WD-40 in/around in the cylinder. Re-install compression gauge and de-choke run starter another 3-5 seconds, then record the reading. Repeat this on all cylinders.

Higher wet reading points toward rings - No change or VERY little change points toward POSSIBLE bent rod from hydrolock, or burnt valves -. (In the latter case, with valve cover OFF you can position the cylinder where both valves are closed, connect an air compressor to your compression tester fitting and see if air is escaping into intake manifold (intake valve), or out exhaust (exhaust valve).

 

We got the spark plug out and it does have low compression in cylinder 2. However, the mechanic I'm working with doesn't seem to want to do the next test. He just wants to replace the engine.

Your suggested Wet & Dry compression test would show bad rings and damage from broken spark plug getting into the cylinder, right?

The appropriate tests would be:
1. Wet and dry test to check if there are problems in the rings
2. Remove valve cover to check for bad valves, intake and exhaust, separately
3. If no problem is found in either of those tests, then the problem is likely the head gasket.
Is that correct?

What do you think? Any other thoughts on troubleshooting?

Thank you very much for your very helpful advice!

 

Is it Cylinder 2 only?

If compression is being lost past compression rings, the WD-40 gets spread around on the ring / cyliinder wall during the 5 second cranking - and will register noticeably higher on compression gauge. It would be anybody's guess what caused the low compression though. But if on TWO separate cylinders like we WERE talking doesn't seem like normal ware pattern. If it is ONLY one cylinder - a very possible culpret could be a bad LEAKY injector can result in partial hydro-lock and bend a rod. That will result in low compression because piston isn't cumming all the way up on compression stroke. Wet test would not make a dramatic improvement in that one. (I caution dramatic because the wet test will almost always be a 'little' (couple or three lbs) higher.

I do not personally think it is a head gasket. 1) adjacent cylinders are not effected. 2) if the leak is not into another cylinder, it would have to be into a water jacket and THAT will ALWAYS result in overheating.

.

 

I’ll probably add some confusion to the discussion but I’m really just trying to understand the facts as I follow along.

First is, are we sure the tech knows the Ford cylinder numbering ?
If he’s a GM kind of guy and used the GM numbering system, then 2 and 4 are adjacent cylinders.

If the Tech used the “built in” PCM and in fact did a cylinder balance test then he didn’t check compression.
It effectively tested the output performance on each cylinder with the engine running.
Low cylinder performance can be caused by any number of things besides compression.

Or did the tech use an amp clamp on the battery cable and measure the “relative” current draw as each cylinder went through its compression stroke? That would show the “relative” compression as well as problems within the starter motors commutator.

What say you?

 

Hi pdqford - good to hear from you.

All good points. Its my understanding were dealing with a 2006 F150 5.4 with 150K miles. And early report was a power balance test ' mechanic diagnosed' as low compression. I think that was confirmed by a gauge test --- ( on Cyl 2 only ,,, ???? IDK why) and I don't know HOW it was done, but apparently not both wet/dry. ?? Maybe that shop owner knows more than he is saying. He said he didn't want to work on it.

 

@F150Torqued, thanks for the update. I'm just trying to get in the right rabbit hole!
Maybe the shop knows more, or maybe they don't know enough

If the shop did in fact do a "relative compression test", then the engine wasn't running, the ignition or fuel system was shut down, and the engine was only cranking.

IF the shop did do a cylinder or 'power balance test ' then the engine would have been running, with the PCM slightly raising the idle speed as it takes turns shutting down each coil one at a time and measuring the RPM drop as each cylinder is shut down. That won't indicate compression, but will show how much power each cylinder is contributing.

Since the list is getting results 2nd and 3rd hand, and you have the plugs out, do your own compression test and post up the results.
If your working with a mechanic, I think you should let the mechanic make his diagnosis and proposed solution. If you don't trust him or think his solution is correct, pay him for his work and seek a 2nd opinion.

Okay, back to silent mode.

 

double post