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  #1  
Old 12-04-2010, 05:19 AM
1989 502,000km 300TE
 
Join Date: Apr 2006
Location: Port Macquarie, NSW, Australia
Posts: 84
1990 300E 24valve low compression numbers

Hi,
Just picked up a cheap 300e 24 valve 230,000km. Wasn't running but the car is in good condition.
The previous owner said when it broke down it started to run rough, then it lost power and would no longer run.
He didn't want to spend any money on fixing it.

Did a compression test when I got it home and the readings are;
1: 40
2: 60
3: 70
4: 70
5: 65
6: 60


Does anyone want to have a stab at what the most likely cause is before I go pulling it apart?


Last edited by Oz 300te; 12-04-2010 at 05:30 AM.
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  #2  
Old 12-04-2010, 07:07 AM
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Join Date: Aug 2002
Location: Southeastern PA
Posts: 3,035
Oz, I would attribute the varied compression test readings to a head gasket and/or valve problem. It's likely you need to pull the head, have it reconditioned and install a new head gasket.
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  #3  
Old 12-04-2010, 09:07 AM
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Join Date: Apr 2008
Location: Richmond VA
Posts: 202
You're using psi versus kPa, right? How did you perform the compression test? I assume the engine cranks over. Did you get four "puffs" on each cylinder (four compression strokes on each cylinder)? What was the reading on the first of the four puffs in that lowest one? It should be at least 50% of the final reading - if not, there's an indication that the rings are worn.

There are really only three ways for the compression to escape - valve leakage, piston ring leakage (or a hole in the piston), and cylinder head gasket.
Did you follow up with a wet compression test? Simply add three squirts (no more than three) into the cylinder and retest; if the first-puff reading is much higher and the final reading is much higher than without the oil, the cause of the low compression is worn or defective piston rings. If the compression reading is only slightly improved, then I'd work on valve testing.

Do a cylinder leak test next.

You'll need compressed air because you want to have a constant air pressure applied to the cylinder. Make sure that the cylinder is at TDC (because then the valves will both be in the closed (seated) position). It is likely that air will be heard or detected leaking out somewhere.

If the air is heard leaking out of the oil filler cap, the rings are the culprit; if you can hear or detect air escaping out the tailpipe, it's probably a defective exhaust valve; if you can hear air coming out of the throttle body or air inlet, it's probably a defective intake valve. If that's all good, undo the radiator cap, and if there's air bubbling out of it, then it's possibly a blown head gasket (or a cracked cylinder head).

You can do all this easily and inexpensively before going to the expense of pulling the head off.

Let us know the results.

My brother in law lives in Mt. Eliza but he won't be any help to you, unless you want someone to serve beers after you've identified the problem!
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  #4  
Old 12-04-2010, 09:48 AM
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Location: beautiful Bucks Co, PA
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Did you hold the throttle wide open during the tests? If you did, check the cam timing. If you didn't, hold the throttle wide open and re-test.
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  #5  
Old 12-04-2010, 11:02 AM
1989 502,000km 300TE
 
Join Date: Apr 2006
Location: Port Macquarie, NSW, Australia
Posts: 84
Hi,
thanks for the replies.
Hopefully I will have time today to do a leakdown test.
And I held the throttle open while cranking.

I checked the valve timing and this is right.

I didn't squirt oil into the bores after doing the comp check.

It doesn't add up to me that the rings are gone as the PSI readings, except for cylinder 1, are all close.

But, if it has been severely overheated then the rings could well be stuffed, but there are no obvious signs that the motor has been cooked.
And it was serviced 4000kms before it broke down.
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  #6  
Old 12-04-2010, 11:26 AM
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Join Date: Aug 2008
Posts: 436
is your gauge accurate? Might want to remove a plug from a good running engine and check it.
It's hard to believe them all being that low- and even.

M
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  #7  
Old 12-04-2010, 12:33 PM
LarryBible
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The odds of such low compression across all cylinders being valve or head gasket related is quite low.

You must now do a cylinder leak down test on each cylinder and while each cylinder is pressurized listen for where the air is escaping. If you hear it through the oil filler hole, it is cylinder/ring/piston related. If you hear it through the throttle body, it's intake valve related, or through the tailpipe it is exhaust valve related.

These numbers, assuming that they are given in PSI are quite low. Typically numbers that low will happen in only one or two cylinders.

Continue your analysis and post the results here for comment before tearing it apart. Look before you leap.
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  #8  
Old 12-04-2010, 05:58 PM
1989 502,000km 300TE
 
Join Date: Apr 2006
Location: Port Macquarie, NSW, Australia
Posts: 84
Hi, I 99% certain the comp gauge is right, but I will double check it.

The car has been sitting for 3 months since it stopped running.

The comp numbers just don't make any sense to me, that's why I asked if anyone might have seen a similar problem as I'm new to the m104.

I have a m103 in a 1989 300TE that has 480,000km on it and when I replaced the head gasket 20,000km ago (because of oil leaks) you could still see the cross hatching in the bores. So I'm surprised that the bottom end on this m104 would be worn out.
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  #9  
Old 12-04-2010, 06:36 PM
LarryBible
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How much has the engine been run after the storage before the compression test was done?
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  #10  
Old 12-04-2010, 10:59 PM
1989 502,000km 300TE
 
Join Date: Apr 2006
Location: Port Macquarie, NSW, Australia
Posts: 84
No it hasn't run at all. I was thinking that the bores may have dried up.

So I just did another comp test, this time I held open the fuel flap while holding the throttle open and the comps came up to 150psi.
It looks like the fuel getting in has helped seal the rings.

I tried to start it and it will crank for 5 sec and sound like it wants to start but then it will start kicking back on itself like the timing is out.

I pulled the plug and it is wet, and the top of the piston is wet so it maybe flooding.

I'll remove the fuel pump relay and try drip feeding the fuel down the throttle while cranking and see if I can get it to fire.
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  #11  
Old 12-05-2010, 12:28 AM
1989 502,000km 300TE
 
Join Date: Apr 2006
Location: Port Macquarie, NSW, Australia
Posts: 84
I think I have found the fault.
I am getting a double firing of the spark, it is intermittent.

so instead of getting
spark...........spark...........spark.
I am getting
spark.spark.........spark.spark..........spark.spark............spark

Could the EZL be sending an early spark followed by a correctly timed spark?

The comps are all around 90psi.
Even though this is low it should still fire up.

Like I said earlier, it goes to start but then kicks back on itself like the timing is out.
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  #12  
Old 12-05-2010, 12:33 AM
home of 4,5,6,8 cylinders
 
Join Date: Jul 2005
Location: Vancouver BC
Posts: 504
were the spark plugs line mixed? i did it once when i put my bmw 320i back together and mixed 2 wires.
unless the timing belt jumped.
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  #13  
Old 12-05-2010, 01:21 AM
1989 502,000km 300TE
 
Join Date: Apr 2006
Location: Port Macquarie, NSW, Australia
Posts: 84
No, the distributor cap has the cylinder numbers on it and I double checked all the leads were right.
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  #14  
Old 12-05-2010, 05:16 AM
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Join Date: Aug 2002
Location: Southeastern PA
Posts: 3,035
Oz, are you sure the coil is good. Our daughter had the same symptoms you describe in her 1991 300CE with the 104 engine. The engine wouldn't restart due to a failed coil ... a cheap and easy fix. That same engine had a failed EZL a few years earlier.

As you're probably aware a new EZL lists for around $2,500. The same vintage 300SL uses that EZL if you're searching for a replacement one. Our local MB dealer's parts system showed EZL's backordered for the 300CE, but available for the 300SL. Took me over a year to acquire a new EZL. Could not find a used EZL during that time.

I apologize for my previous brief post, but as far as I know the bottom ends of the 104 engines are bullet-proof. So I didn't think a 104 engine with low mileage could possibly have piston-related problems.
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  #15  
Old 12-05-2010, 06:19 AM
1989 502,000km 300TE
 
Join Date: Apr 2006
Location: Port Macquarie, NSW, Australia
Posts: 84
The coil I think is still ok as it is supplying a strong spark.
I did a search on ebay for all the compatible ezl modules, there are about 8 according to MBDOC.

They are all around the $700-900 range.

I would install a megasquirt rather then spend that money on a second hand ezl.

Only thing is the harmonic and pulley are different to what is on my 300TE and they appear to be one unit, that is going to make it a bit harder to mount a trigger wheel. I'll have to weld it on.
See these pics.


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