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PCM update - needed or just guessing?

2002 Ford Ranger Edge 3.0 Flex, 118k miles. 3 months ago, had a massive vacuum leak b/w the PCV and intake manifold. Didn’t find it right away, and in process drove it for about 1000 miles while replacing 2 upstream oxy sensors and MAF per engine codes. Vacuum leak was severe, but no one told me to look for it, just sold me parts. Lesson learned. Ran fine for about 2000 more miles after that, and now have 0304 code for misfire on cyl 4. Replaced plug wires, replaced coil block (returned it when it didn’t help), finally had it scoped (per advice) and they say it may need a PCM update. SInce this involves no hard parts, I wonder if this will fix it once and for all, or another wasted effort. It starts rough, has low idle speed, and pings. Once fully warmed up, I can shut off for 15 secs and restart smooth. Stays smooth until I let off the gas and allow wheels to push engine, or if I allow rpms to drop rapidly from running to neutral/idle (5-speed). So long as I am applying power to the wheels and shifting, stays smooth when warm. Engine light sometimes flashes, sometimes steady, sometimes puts itself out, sometimes I have to clear it.

Forgot to mention that I had it compression checked recently, and it got new plugs at 100k. I’ve also been adding one can of B12 to one tank of gas per year since '04.

What were your compression readings, because it sure sounds like a massive misfire.

Compression was fine, according to the guys who checked on it. They ruled it out and went with the PCM recommendation, but they couldn’t be sure that would clear up the problem.

I would still want to know the actual numbers on the compression. Many times compression is stated to be fine when it’s really not; and this is even done by mechanics.

Anyone who runs a compression test should always put those numbers down in writing; preferably on the customer’s copy of the repair order.

I’ll see if I can get the numbers. It was checked by a third party working for the body shop I took it to for a minor ding the other day. Got them to send it out to have the engine investigated as part of the body work.

I looked hard at your post and did not see where you’d replaced the plugs. Are these the originals? If so, you’re overdue anyway, and sometimes the pronlem is as simple as that.

New plugs last year at 100k. The 3rd-party repair shop didn’t write down my compression readings. sigh

For the first day since I got it back from the body shop, it still had the engine light acting as I’ve previously described. For the past 2 dyas, the light has not come on at all, while the rough engine continues exactly as described. At this point, I have no idea what to do. I haven’t got unlimited amounts of cash to throw at this, even though every mechanic I deal with seems to think I do.

Body shop? Is there something you haven’t told us? An accident perhaps?

Look at the post just prior to your previous one…

Got it.

I got hit in the rear quarter a couple weeks ago (other guy’s fault), and took it to a body shop. When negotiating the repair, I asked if he could do me a favor and have the engine scoped. He said yes. What I didn’t learn until after was that he sent it out to some third-party place and verbally relayed what they told him - or so I was led to believe. However, that data and advice he gave me went out the window when I bit the bullet and took it to the dealership this morning and had THEM scope it and do an evaluation.

Everything the body guy told me got overturned. Ford said that compression on 5 of the cylinders was fine, and that #4 was 22% low. He said it might be valve guides, but is most likely rings. He stated categorically that a PCM update would not have been any help at all, and that a rebuilt short block would cost me in the ballpark of $5500, so I’ll be doing that repair myself.

You guys were spot-on: it was all about the compression data, and I am SO glad you got me to doubt that second-hand oral-only report enough to have it competently re-checked.

I just hope I do a good job rebuilding it. Last engine I rebuilt was 25 years ago on my '68 Mustang’s 302. Things are different now, I understand, and I am not nearly as strong or flexible as I once was. Still, paying someone else to do it is out of the question. I’d have to buy a bicycle and find other work.

THANK YOU ALL for your correct guidance, and any further advice before I get up to my elbows in engine would be very much appreciated.

I don’t see how a PCM update can fix a problem that didn’t exist just a few months ago. Seems unlikely this would fix your problem. Ford experts likely know better than me though.

Here’s my first thought: Your symptoms could be a fuel mixture problem. The rail fuel pressure is probably regulated by vaccuum. Maybe something is still not quite right with that still. Since you are still having drivability problems, it might be worth it to have the rail fuel pressure tested. Make sure they verify the fuel pressure changes with applied vaccuum per the Ford specs.

Did the Ford garage actually measure compression by removing the spark plugs and using a compression gauge or did they calculate the compression based on the “scope”? I would not trust the “scope” method. It simply measures the power produced by each cylinder and if one cylinder is 22% low, they assume that it is due to compression.

A PCM update would not fix a problem that did not previously exist. I would even question if an update is even available. You can do a reset by disconnecting your battery for awhile. The time depends on which model, it can range anywhere from 10 minutes to 24 hours. Clearing codes is not the same as a reset.

If you do have a cylinder with low compression, it will probably be due to a burned exhaust valve, not rings and certainly not valve guides. You have the 3.0 engine which I believe is a Mazda engine, not a Ford engine, so it may have solid lifters. If it does, you should at least check the valve lash before doing anything else.

You guys seem to know what’s up, so I’ll check these things out one by one as I am able before tearing the whole thing down.

I do not know how compression was checked, but I have two nicely printed graphs here. One says “Power Balance” and shows 652 RPMs, and shows cylinder 1 at -2 or so, cylinder 4 at -68 or so, cylinder 2 at about +25, cyl 5 at +50, cyl 3 at +25 and cyl 6 at +2.

The other graph is called “Relative Compression” and lists Mean Cranking RPM at 234. 5 of the cylinder bar graphs show 0% difference, and cyl 4 shows 22%.

Oh, and yes, I did disconnect the battery for about 5 hours two nights ago, so that’s probably why the check engine light has not come on since.

Today, a friend of mine recommended I dump a 6-cyl-sized can of “Restore” into the fresh oil change I did a couple days ago, and darned if it isn’t running a heckuva lot smoother. It still starts rough when cold and smooths out when fully warmed up, but it doesn’t go rough when I let off the gas in gear any more and it also doesn’t drop into rough mode when I let the rpms fall rapidly to idle, so it’s making a big difference already, after just about 50 miles or so of driving.

So now I’m left with the roughness when the engine is cold.

Lifter lash, burned valve, and rail pressure you say? It may take a while, but I WILL get back to you on those! My old comprseeion tester for the Mustang won’t fit these plug holes, so I’ll have to borrow one somewhere to start.

You guys are aces! Beers are on me if you ever get to Phoenix!

Phoenix? Too hot.

A power balance test can be done with a scan tool. An injector or coil can be turned off one at a time and the scan tool records the RPM drop. This will indicate something is wrong but does not prove it. A hands on compression test must be performed.

Has anyone removed and inspected the spark plugs? A lean condition can burn the tip off of a spark plug.

I replaced plugs 18000 miles ago, but yes, I personally inspected #4 plug and it showed no appreciable wear or coloration. Gap was a little closed according to spec (found it at .040) so I regapped it to .044.