Low on compression test but ok leak down test?!?!

[tryingbe]

Ran a compression test, HOT. Wet test made 5 to 0 psi difference on two cylinders. Tried two compression gauges.

80, 75, 75, 80

Test leak down on #4, #3, #2, #1 less than 2%.

18 inch in vacuum at idle, no oil on plugs, cam/ign timing is correct, no blow-by, no ejecting dip stick. Only sign is poor mpg.


Rebuilt 2.5L with Ross pistons, o-ringed block. Less than 1000 miles on engine.

Mopar 2007 head gasket.

PT lifter with 2 washers each

Unknown used roller cam shaft. Could be regrind, could be stock, don't know and no way to tell.

Coolant is clean, oil is clean and smells like oil

Help?

[22mopar]

there are basically only a few causes for all cylinders to be very low on compression....incorrect cam timing, all cam lobes worn out, massive exhaust restriction.

You say cam timing is on, but I hear that one a lot and 90% is not right. I know you are competent but just never rule out the obvious. Just the other day, I was off 1 tooth on my race motor. It seemed lazy on the top end. when I re-checked cam timing, I was off just one tooth.

[tryingbe]

Any ideas that prevent me to pull my hair out is good. I welcome all suggestions.

After putting the stock cam cover back on, I might be couple of degree off on my adjustable pulley. Will set it and report back.

[butchsuppe]

1 tooth off.

[tryingbe]

Adjusted a few degrees, # still low at 75 psi.

I don't have a spare cam shaft here, but I guess I will find one.

[x.Gen]

honestly, hard to tell from those angles but that looks off a tooth retarded to me (I know you've got it advanced at the pulley). on stock VC's at 0' cam timing, mark usually falls centered between the middle two ribs if I remember. you've got it advanced 3' and it looks like it is straddling the second rib. if you are off a tooth, that is like 6' retarded (which would eplain the weak compression). my 2.5 loves a few degrees advance, when cam is retarded it runs really flat and gets bad mpg.

advance the cam further and do compression test. if you're off a tooth, you'll need 10' advance to get compression up proper.

---------- Post added at 01:53 AM ---------- Previous post was at 01:51 AM ----------

forgot to mention - put a timing light on the cam pulley, that always helped me.

[RoadWarrior222]

Umm if this is static compression, what difference would timing make????*

Causes for low static compression, would seem to me to be... pistons dished more than stock, fat O-rings and/or headgasket, hogged out chambers (Overenthusiastic "porting")

What were the numbers immediately after rebuild?


edit: * Well I guess I'm thinking you'd turn motor until you saw the gauge peak, rather than turning it to theoretical TDC and expecting it to be peak... Or I guess, I'm thinking that the timing would have to be hella way off for the valve to be open much into the compression stroke, which might not be the case.

[tryingbe]

Sorry, don't have the # immediately after the rebuild.

[shackwrrr]

those are about the numbers I got on my 2.5 that pulls 21 in of vacuum, no blowby, and no power issues. Did you have the throttle body open? I didnt on mine.

[tryingbe]

WOT. No piping was disconnected.

Power was fine. Only real issue was I couldn't get more than 21mph average in an Omni.

[shackwrrr]

I was pulling close around 32-33 in my shadow on the highway. Sounds like you might have an issue elsewhere or those ross pistons have crazy dish and put you into the 7's for compression ratio.

cranking compression is usually 10-20 times (depending on cam overlap/timing) the compression ratio. If your pistons and dish put you into the mid 7's for compression ratio and your cam has some overlap or is timed wrong you could get 80's for cranking compression.

[x.Gen]

Umm if this is static compression, what difference would timing make????*

if cam timing is retarded enough, valves are closing too late to get decent compression.

Power was fine. Only real issue was I couldn't get more than 21mph average in an Omni.

you have a bunch of variables with the unknown cam & those pistons. if you are logging or have the CEL active in your cal, after getting a better handle on the cam timing I would think about more aggressive ignition timing. when I ran my 2.5 on the 2.2 GLHS MP II cal (SOOO wrong), I got 23 mpg in a G-body when I frequently hammered it. proper cal bumped it up substantially. you should be getting better mpg for sure, unless you're WOT everywhere.

[tryingbe]

Found this interesting way to measure the shim needed for the lifters.

http://www.turbo-mopar.com/forums/sh...l=1#post396395

[shadow88]

Remove the shims and pt lifters and put the stockers back in. Just something else to rule out. I could see the hydraulic pressure of the oil pumping up the lifter to the point that the valve remains open.

[tryingbe]

Put a 88 TBI 2.2 roller cam shaft in, retarded it couple degree until the cam is as close as TDC I can get. Compression # is now less than before with the unknown camshaft. The widest lobe on the unknown camshaft is 14.72mm, TBI cam shaft is 15.91mm.

[x.Gen]

ok, first, correct me if I'm wrong - low leak-down, even compression, STRONG pistons. good to put a beating on without worry, right? second, I'm trying to get why you set the TBI cam to retard before testing (what, because it is a TBI cam? try running it straight up regardless of pulley type). I always do compression tests with the cam straight up. unless I have no clue, retarded cam timing will only result in lower numbers. are you setup for datalogging? there isn't a lot of info on the Ross 2.5's, unless Gary will chime in. as I understand the available info, lower compression isn't a huge surprise, but with the info you've put out there it isn't horrible news either.

[tryingbe]

ok, first, correct me if I'm wrong - low leak-down, even compression, STRONG pistons. good to put a beating on without worry, right? second,

Yes, those the facts so far. Power is good, ran a 14.5 at 97mph at 12psi. MPG on the other than, I'm getting 21mpg average, and I don't usually drive over 65mph nor accelerate that hard. I understand the GLH has drag of .44, but it's 2500lbs with driver. My Turbo Volvo wagon that weights 3600lbs is getting 25mpg with a/c on!

I'm trying to get why you set the TBI cam to retard before testing (what, because it is a TBI cam? try running it straight up regardless of pulley type).

I thought the cam key is the center line of the cam and should be pointing to the directly North of the head? I'll do a compression test with a straight on, and do it with 4 degree advance as well and see what # I get.

are you setup for datalogging?

I have a OTC 4000e. Will making the cable and get a USB to serial converter very soon. What data do you want?

there isn't a lot of info on the Ross 2.5's, unless Gary will chime in. as I understand the available info, lower compression isn't a huge surprise, but with the info you've put out there it isn't horrible news either.

PMed Gary, and hopefully he reply, but he hasn't logged in since May.

[Juggy]

the cam looked more like 2 teeth retarded to me.
poor MPG is prolly cal related. you just dont have the proper timing under vaccum

[x.Gen]

forget due north. starting with the cam at 0' and properly aligned at the belt is the method I would use. from experience, I wouldn't play with cam timing without first starting out straight up (especially for compression test), and datalogging to see results. you'll have a real hard time determining results from cam adjustments by the seat-of-the-pants dyno. retarding it is masking the proper results if you ask me.

only asked about datalogging to see where you are at in the process. once you're set up, I'd monitor knock for timing adjustments (and if your compression is as low as you say when timed right - I'd be jacking up the timing curves). these days, Gary seems to keep himself busy months at a time, and make an appearance every now and then.

tell me you aren't running a GLHS module in there. read my ostrich thread from early last year. I don't care what other people who've run it say - a 2.5 block on a 2.2 cal is not only unsafe, but extremely unsatisfying. trust me, I did it. the right cal changed the car into an absolute animal. a 2.5 in an omni should be averaging 25 mpg plus based on my g-body experience.

[Juggy]

well once that cam gets advanced 2 teeth then im sure MPG will def increase.....car will also be alot peppier. i never did a compression test on my 2.5, but it had a G head and taft S3 camshaft, when i rotated the engine the compression was so low that i hard a hard time feeling out TDC without actually watching for the timing mark on the flywheel. the car ran like a beast tho, wish i had got it too the track the one chance i had b4 it got dismantled