low compression, but why?

[quest49s]

Here is a little background, I was cruising to work going 40 in 3rd so around 3.5k rpm and car ran great. Car Pulled strong and had plenty of power until, I Pulled up to a stop light and took it out of gear and it died, then i tried to turn it over and it wouldn't start. towed it home last night, I checked everything (spark, fuel, ecu, msd, CAS, ...) figuring that there is no way the compression could have gone straight down the toilet so fast.

I did a comp. test last summer ('03) and the numbers were dry ~160 (+- 5) across the board at 5k feet above sea level, have since put about 4k on the rebuilt motor (then brandnew ~100mi) and here are the numbers from tonight:

cyl 1 2 3 4
dry 75 75 75 75
wet 100 110 110 95

This is very odd to me because the rings just don't all go bad at the exact same time and have the same compression in every cylinder. I also let the car warm up fully before I drove it to avoid piston slap w/ JE's

My timing is dead on so I wouldn't think i smashed the valve train, and also didn't hear any uneasy noises while driving before the episode.

A head gasket probably isn't to blame either as the comp. would likely be different per cylinder.

I wouldn't think it to be any sort of crank walk cause it's a six bolt but I never had any of the symptoms that a crankwalking 7 bolt would have.

Does anyone have any clues as to what is up?

Motor Stats:
'91 6 bolt w/ 4k mi
Rebuilt last winter w/ the following:
JE 9:1 pistons ceramic coated & teflon skirted
Eagle rods w/ arp hardware
Stock crank
balance shafts removed
new front case and oil pump
Mitsu 4 layer head gasket
Arp Main studs
ARP head studs
bone stock head
ACT 2600 lbs
RRE flywheel

Thanks,
Jon Hebbeln

 

[crankbender]

With that ammount of wet change and the fact tha tyou are running 9:1 it just about has to be the rings. Not sure what caused the problem....have you been having oiling issues?

 

[1gE.T.]

Have you tried putting oil in the cylinders? If not put about a half an oil cap full in there and check again. If the numbers go up then your rings are shit. If not then its something else like headgasket or the valves arent seated right or something.

 

[Mike 99GSX]

Originally posted by 1gE.T.
Have you tried putting oil in the cylinders? If not put about a half an oil cap full in there and check again. If the numbers go up then your rings are shit. If not then its something else like headgasket or the valves arent seated right or something.

I think that's what his "wet" numbers indicate.

 

[dls93tsi]

I had the same thing happen to me in August, my timing belt tensioner failed but the motor didnt die until i took it out of gear and the timing belt lost tension. If it is the belt, bent valves would prbably cause the comp. drop. pull the timing belt cover off and see how bad it looks.

 

[quest49s]

I fear that it may be rings aswell but it is just odd that compression dropped all at once and it is exactaly the same dry comp. in each cylinder.

The car pulled very strong w/ no detonation, had plenty of low end torque and then boom compression went from mid-high hundereds to 75 psi. I just can't seem to make sense out of it all?

I have been having oiling issues with an oil leak on the oil pump sproket leaking past the seal. But my oil pressure is well abouve standard (100+psi at any rpm higher than 4250)

My timing is dead on.

 

[1gE.T.]

My bad, must have overlooked that.

 

[quest49s]

I guess the only sure way to tell is to get a leak down test?

 

[Zenja]

Maybe it's not an engine problem. Maybe something is not letting air in/out of the engine.

Maybe it's something with the cams.

 

[nineD5_GS-T]

Very rarely does each cylinder wear down that evenly(75 across).

First you were @ 160, now your at 75. Does the car feel any different.

I would suggest borrowing a different compression tester. Maybe your tester went bad.

 

[quest49s]

Thats a good call on the cams, I'll swap in a the pair from my spare head. I din't even think about that possibility. if something was't letting air out of the engine(just in) however that wouldn't have the ill effect of low comp, correct?

The comp. tester went up with the wet test though, non the less it could be a faulty tester.

The car was running perfect felt the same as the day I started it, with the new motor, until it died, which is very very odd. It still had plenty of low end torque, and spooled my 18g to 18psi at 3,500 rpm w/ stock downpipe. The car was tuned for no detonation on 91 oct. but was running rich. EGT's never even got to 1450 on WOT runs to 6k in 4th.
In fact I demolished a 13.5 sec. mustang 3 days before this episode in street setup.

I'll do a leak down test (if the cam idea falls through) on Friday and get back to you guys

If any one else has any input feel free to tell me.
Thanks,
Jon Hebbeln

 

[crankbender]

As weird as this may sound do you have an egr blockoff on the car? If not you could have clogged the intake or exhaust at some point and not be getting good valve seals. If you put in too much oil the oil got the carbon sticky and increased your pressure that way.

To fix this start the car and run it to 2k rpms. Take off a vacume line and just keep sucking that cleaner into the engine (do a search for what to use and all).

Also make sure you clean out from in front of the throttle body.

This is unlikely but it only costs a few bucks, is quick and easy, and who knows.

I don't think that cams could have anything to do with it changing that much when it was wet. It really seems that you are loosing something somewhere.

(is your battery low)?



All taht said this has nothing to do with the original problem. The engine should run still at those pressures and deffinately run once it is started. It may be that you did plug your intake somewhere.

 

[quest49s]

Yes I do have an egr blockoff plate. This does not appear to be the case however, even so I can not start the car. I did this about six monthes ago to attempt to ride the je's of some of the carbon deposits fromrunning rich, i used an 1/8th of a can of MCCC(Mopar Combustion Chamber Cleaner) per cylinder and sucked the rest through a vacuum line as you said.

Regardless I cannot start the car.

If I did plug my intake it would have to be a pretty air tight clog to not allow any air into the cumbustion chamber and thus create a vacuum where there is ~%50 less air then absolute pressure.(14.7psi or up here~11.8psi).

Granted when you do a compression test and the TB is closed as oposed to open is only about 10 psi. The TB is a fairly large restriction when closed. But not large enough to create a vacuum where there is no air. To create something like that there would have to have a few rags stuffed in the UIC pipe, if not in each runner of the intake mani. or serane wrap.

Jon

 

[crankbender]

I was saying the clogged intake could keep the car from starting and the sticky valves might not be closing tight. not the other way around .

Lets try and solve the starting problem first. You should fire on that compression even if it isn't right.

1 Do you get any fire at all or does it just turn over and over?

2 Does it turn over slowly?

3 Do you have spark?

4 Do you have fuel?

 

[quest49s]

1. no just keeps turning and turning
2. No
3. yes: have fuel pressure and fuel on spark plugs when taken out
4. yes: sparks at TDC via Timing light

 

[crankbender]

I do have to say that if you have spark and fuel and you are getting nothing at all something is wrong. I can't believe that you are below ignition point here. Basically your car should start crappy maybe but it should run.

Do you get it to fire on any cylinder at all??? like even once as it turns over?

 

[Zenja]

Well really it has to be something to do with air. MAYBE spark too; does a timing light show when the engine is thinking that it needs to fire, or when the spark actually fires?

I really think it has something to do with air if all your observations are true. This means that either air isn't getting to the TB or that the intake valves aren't opening.

 

[V8_SLYR]

Here is another idea.... on my car I had a sticking HLC hydrolic Lift Compensator (others call them lifters) it pumped up on the exhaust side and was keeping the valve open. When I did compression I would get 160,50,160,155 so there was something weird. I don't think yours all stuck open at once thats two wierd but it is another spot to check. To check them pull the valve cover off, take a screw driver to compress he spring a bit then remove the rocker arm and slide out the HLC (note the cam has to be in the right position) once they are out you can put it in a vise and slowly compress it (look for air coming out the oil hole... if so thats not good). Once it is compressed reinstall it and try you compression again. Another note is once it does start up again after this its kinda loud because it takes a while for the HLC's to pump back up again. This is a shot in the dark but just another Idea to consider.

 

[Mike 99GSX]

HLA= Hydraulic Lash Adjuster. Just to get geeky technical

 

[Crazy-MC]

I have the same damn problem I'm on my third engine rebuild with the same numbers as him! I had the block taken to the machine shop, ring gaps checked, crank checked, everything that I possibly could Fk up I let them do. Now this weekend I'm putting a reman cylinderhead on from national cylinder head. I'm praying this works!

 

[Zenja]

Just so you know about those wet numbers, you're probably getting higher readings when wet because the oil takes up a certain amount of space in the cylinder and effectively increases the compression ratio. I know because I accidentally put too much in while doing a compression test two days ago and it came out to 280 psi

Maybe your gauge is bad and a different problem is preventing you from starting the car? It's quite possible.

I think the most likely things by far is something to do with getting air in. There's really on other way that every cylinder could be compressing only half as much as it should.

 

[Crazy-MC]

That is very true! In fact how in the hell would the oil get on the cylinder walls when your pouring oil through the spark plug hole! It just collects in the dish of the piston. But any way any results on the compression issue?

 

[thejeebus]

Here' a suggestion:

I've never had forged pistons in any of my cars but I do know about the expansion properties present in them. How long after it died did you do a compression test? I would think if the engine was cold and had forged pistons your compression results would be lower than a cold engine with stock pistons.

This is just a suggestion. Your car should still fire though like others mentioned, I would check the CAS, Possible BIG vaccum leak, and I would check to see if the fuel pressure is up to par.

Scott

 

[Crazy-MC]

If you want to get your car started, clean the plugs, then start the car with the gas pedal held down. This prevents the combustion chamber from flooding and makes the computer go into a special mode. Don't pump the pedal. I have been using my car with the same compression for the past three months. I'm using stock pistons. But let me know what is causing this low compression.

 

[Crazy-MC]

IT LIVES!!!! The pressure is awesome! For 400 beans my three and half engine rebuilds have resulted in sucess I think it was the hydro lifters. But between eliminating all the variables and running back and forth between machine shops and autopart stores the purchase was well worth it! Hey Jon if you haven't parted out your dsm yet try swapping out those lifters! They're only a 100 bucks. DSMotorsports sells them! Now that I have finally reached stock running order its mod time Let me know how things work out Jon!