Can anyone give me a good ballpark figure on compression in the 5.0L EFI? All the stinkin Haynes manual says is that the lowest reading has to be within 75% of the highest. I ain't no rocket scientist, but that's pretty easy to figure out by myself. I got numbers ranging from 180 - 205 when I checked mine today (Warm Engine) I let it sit just long enough to let the headers cool so I could keep most of the skin on my fingers and hands. I've noticed a lotta folks talking anything from 90 -130. Is 180-200 high? Too High? All of the cylinders built max compression pretty quickly, within 7 or 8 strokes. I turned the engine til I saw no movement on the gauge, but none of them ran more than about 12 strokes. Instructions with the gauge and in the Haynes manual say at least 4, but didn't put a max number on it.

Engine's got 136K, but mostly highway miles cruising at 70 - 80 mph. What's got me doing a compression test is I have a miss under load. Lower gears ( 1, 2, and 3), light-medium throttle, all is OK. Heavy throttle (climbing a hill in OD with cruise on) and it starts with the sputtering and missing. Cleans right up as soon as the throttle backs off. Depending on how long the climb is, it'll actually develop a dead miss on at least one, if not two cylinders. This is driving me nuts!! Get no codes, have replaced every sensor except Knock Sensor and TPS. New wires, cap/rotor. New Bosch Platinum II plugs. Recently R&R'd throttle body/plenum and replaced gaskets. Replaced O's on Injectors while there. KOEO and KOER both indicate nothing wrong with TPS or Knock sensors, and I've checked the TPS with a meter. Nice smooth linear signal bottom to top. .95 V at idle, 4.5V at WOT.

Here's the numbers from today's compression test:

#1 195
#2 180
#3 190
#4 190
#5 195
#6 200
#7 200
#8 185

Next, can someone point me in the right general direction for info on doing a leak-down test?

I've run every test I can think of now, except a leak-down check. Everything keeps coming back positve, and by the results from all this testing, the truck should be running like a scalded dog. Instead, it's running like a 3 legged cat on a marble floor.

 

I can't comment on the other things, but those compression numbers look good. I sure wouldn't worry about them being too high. Did it miss before you installed the Bosch +2's?

 

http://www.xs11.com/tips/misc/misc3.shtml

 

The compression seems fine. It's mostly a matter of all the readings being within a certain percentage of each other rather then a certain figure. If one or two were much different from the others, I'd be concerned. What you're describing sounds like an ignition problem. I'd check for bad plugs, wires, or corroded cap for starters. I've always had good luck with autolite plugs and have heard some bad things about running platinum plugs in a 5.0.

 

Thanks for the posts, guys.

Yes, it had the miss before the Platinums. I still have the Motorcrafts that were put in last year with cap and rotor by a dealer. After 30K+, the Motorcrafts seemed a bit too clean to me, like it's running very lean. The Platinums improved idle and low power operation some, but only after I fixed a real problem. (See my gallery) That cleaned up the idle pretty good, but now I'm thinking I should have spent a little more time on the injectors. After I replaced the plenum and TB gaskets and cleaned both, I cranked it in my garage at night and turned off the lights (old trick I learned with my other toy). Yup, gorgeous light show, but not very helpful for efficiency or driveability. New Motorcraft wires fixed up that problem. Since the 92 OBD-I uses bank firing (4 at a time), my guess is that light throttle or idle is gonna do just fine. Just when I really start sucking some air in, it starts with the missing. Under sustained load, at least one cylinder drops out completely until the load is removed and the throttle backs off. I've checked and rechecked the timing so many times my 13 yr old step-son knows how to do it. (He's good help)

So, I got to thinking burnt exhaust valve, but the compression numbers argue against that. I keep getting pointed back to the injectors or the cats. They're both OE with about 137K. The cap and rotor only have about 36K on 'em. MAP, ACT, ECT, all replaced. TPS, EVP both check good. O2 installed about 40K back when I first started having the poor mileage issues, and was donating regularly to the local Ford dealership. At $80/injector, I just don't have that kinda cash to drop on the truck now. I'm under the impression that the ECM is compensating for the lean spark-knock by retarding the timing, which would explain the power loss. After the plenum repairs and TB cleaning, it's lot better, but still needs some help. Guess I'm pulling the plenum off again to yank the injectors and do a better job on cleaning them. It appears I had a combination of a lot of problems, but really, just one big problem. Old age.

Now, you guys ready for the really weird part? I can disconnect the O2 sensor, and there's NO change in the way the truck runs, and I get NO CODES. I begin to wonder if that buncha knuckledraggers at the dealership broke the wiring to the HEGO, or somehow disabled it in the ECM. I've driven it about 500 miles since I fixed the plenum, and like I said, NO CODES. Well, save the ones that say in the words of James Brown "I FEEL GOOD!!". I would have thought that I'd get a code if I unplugged the HEGO and drove around some, especially since the engine was already warm when I did it. Definitely get one if I disconnect EGR sensor.

I've had one very quirky problem with a coil before, and saw something then I wouldn't have believed if I hadn't seen it with my own eyes. The rubber hose from the brake booster to the plenum was laying on the coil, and the coil was arcing to it. Yes, to a rubber hose. Go figger. A tywrap fixed that right up, but the coil still died shortly after that. That went all at once though, as coils tend to do.

I heard something squealing, and isolated that to a leaking EGR (vacuum). I fixed that up by blocking off the EGR until I can get rid of the load miss. I just have to keep reminding myself, this truck STILL has the OE brakes on it at 137K, so it's due some TLC.

When I get this one straightened out, I got an 86 T-Bird/3.8L, C6, with a much more random problem that's really gonna be fun. That too says it feels fine, but I have an intermittent problem that only shows up when the engine is warm, but not at highway speed temperature. Cruising around at 30-50 mph city traffic, light throttle or float, it's like the injectors just turn off. WOT, it'll take off like a bandit. But, normal driving, it's very annoying at the least, and can be a little dangerous when pulling out in traffic. I may pull the injectors and clean them (only 2 of 'em on the TBI). If I get on the interstate before it gets to operating temp, everything's just fine. Then, when I get back off the interstate and cool back down a little, it starts acting up again. I'm wondering if I need a hotter t-stat in it to delay the closed loop swapover a little. Certainly won't hurt to swap out the O2's on it.

Thanks again, and I'll quit worrying about compression.

 

It's possible that the EGR is carboned up, and not closing fully. It's also possible that the cats are toast. Pull the O2 sensor and look at it, is it all black? If it is you have a rich problem. It may be within limits in the computer so it may not throw a code. It almost sounds like the computer isn't coming out of "open loop" mode. This is the mode where it ignores the O2 sensor. What temp thermostat is in it? It needs to be 195 degree for the computer.

 

EGR is currently capped with a metal gasket I made from a soup can. It's outta the picture right now. This is a temporary fix to a known vacuum leak around the seal for the operating shaft. Don't have to have EGR, but it helps a little with economy if everything else is good. Economy needs a lot more help than EGR has to offer, and blocking proves it's not the source of the miss.

O2 sensor is clean, too clean, perhaps. I pulled the OE out last year, and it was clean, and the new one I put in is still clean. Definitely not running rich. This is also indicated by the spark plugs. They too looked too clean when I pulled them.

Still has the OE t-stat in it, as well as OE hoses, radiator, water pump. Nothing has been done to the cooling system. Coolant has only been changed once since I've had the truck. I got it used, with 16 miles on it. I'm 2nd owner, but OO only had it long enough to drive it home and back to the dealer. I'm assuming the T-stat is 195 if that's what the assembly plant put in the engine. Temp looks good, about the same it's ALWAYS been. But, that could very well be part of the problem. Dunno how age affects a thermostat. This could very well answer why I'm not getting a code when I disconnect the HEGO and drive it. The truck's always run a little on the cool side, NEVER above half scale on the gauge. But, it has run very well before, and has gotten 22 MPG. Not bad for a 5.0, M5OD. I might go get a new t-stat (200?) this weekend and try that out.

Cats being toast, another possibility, albeit, they don't appear to be blocking much exhaust. Good exhaust flow from tailpipe, no rotten egg stench, in fact, the exhaust has almost NO smell. However, since I had shorty headers put on it, they've been buzzin and rattlin at low RPM. Probably should pull the muffler and see if there's any fragments in there.

Thanks for the input. Gives me a couple more things to check in depth. Avoiding pulling the Y-pipe/Cat assembly. Gotta pull the tranny crossmember to get it out in one piece. I thought about rodding out the cats, but the right bank down pipe is a bit too crooked to get anything straight in the front one. Could probably knock out the back one pretty easily.