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Old 10-24-2007, 10:43 PM
Registered User
Join Date: May 2006
Location: Hudson WI
Posts: 178
103 Engine Compression Test Results

I bought my 1989 300 SEL off the internet sight unseen. Ya I know; I知 an idiot. Believe me; I値l never do that again. Looks great, low mileage (91K when purchased 1.5 years ago), and one owner, but lots wrong with it. I知 now at 125K, hopelessly upside-down in the thing and it痴 starting to make a clacking sound like a cam or lifter is going. I do like the car. It痴 a great looking car, black on tan, but it is getting some rust. Interior is mint.
With all that said, I知 contemplating yet another large investment into a new cam at the least and possibly a top end rebuild. I live in Northern Wisconsin and the weather is rough on the cars up here. The body continues to degrade in spite of the beautiful interior and potential good mechanicals. I did a compression test with the following results in numerical order starting with #1; 165, 167, 155, 155, 175, 165. Can anyone help me interpret these numbers? What should compression be, and what kind of variance is acceptable? I know there are a lot of factors that influence compression. How hard is it to change the cam? I guess my point is if the rings are also bad, it may just be time to cut my losses and move on.
Thanks for any ideas.

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Old 10-25-2007, 12:25 AM
TheEngineer's Avatar
Registered User
Join Date: Sep 2007
Location: West Seattle, WA
Posts: 89
I think the book says"within 15%" I'd say within 20 lbs. If the rings are bad, you can test for that by putting a squirt of engine oil in: If the compression comes up significantly like 20 lbs it's rings. Valves are usually leaking, but it's easy to do the head: Just take it off and bring it to a machine shop. Watch it: They like to surface the head. Don't have them do it if the head is not warped much. Measure the thickness several places: It should be parallel. If they machine it too thin, it's scrap. It's aluminum and will bend easily .010/.015 inch when you torque it down. They are all curved when they come off. The gasket is quite thick and will take up some uneveness. Check your valve clearance before you do anything.
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Old 10-25-2007, 05:05 AM
t walgamuth's Avatar
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 38,592
whoooah there pardner. YOu may be jumping to conclusions. The compression doesn't look that bad to me. What kind of oil are you using and did you switch brands lately? The hydraulic valve adjusters on that engine are very oil sensitive. If you happened to switch to a thicker oil it might be just that alone making a tick.

On my 84 500sec when I first got it I put in rotella on my first oil change and immediately got a hydraulic lash adjuster pumping up slowly and making a valve train tick. I switched to mobil one 10w30 and the tick was gone forever.

Before pulling the head I would try mobil one 10w30 and see what happens. The compression is within what I would consider normal variance, 20% which would be over 30# variance allowable.

Tom W
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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Old 10-25-2007, 09:56 AM
RBYCC's Avatar
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Join Date: Mar 2007
Location: DELAWARE
Posts: 1,041
Compression looks good...well within cylinder to cylinder tolerance.
I had 190-195 psi on my 63K mile M103-12V prior to a twin turbo install.

The "clicking" noise sometimes is normal.
You can run a can of valve lifter cleaner through the engine, drive it a bit then change oil and filter.
I currently use either Castrol Syntec or Mobil One 5/30 in my twin turbo.

Recommend at oil changes you use a can of Chevron techron in the fuel tank.
Keeps the injectors clean and performs a bit of valve decarbonising.

Ed A.


Check around your exhaust flanges, possibility of a minor exhaust gas leak which can sound a bit like a lifter noise !
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Old 10-26-2007, 11:19 AM
Registered User
Join Date: May 2006
Location: Hudson WI
Posts: 178
Thanks for the replies guys, I appreciate it.
I致e been running Mobile 1, 10-30 since I got the car, with regular oil changes. I've had no valve train noise until recentley.
I was thinking the clack might be a lifter, but I致e heard so much about the 103 soft cam issues that I assumed I値l find a bad cam in there. I am going to pull the valve cover this weekend and see if I can spot a bad lobe. If not I値l try the lifter oil treatment as was suggested. Thanks for the great tip on the oil in the cylinder to verify ring seal during a compression test. I知 going to check out the rings before moving forward on any rebuild. Thanks also for interpreting my compression test results for me.
If I do need to replace the cam, does anybody know if I have to separate the timing chain to remove it? Or can I just tilt it out of there?
Thanks again.

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