Kinda scratching my head on this one. The car would keep losing coolant and I would see it come out the overflow tube on the expansion tank. Upon further inspection the weep hole on the water pump was also dripping. So I replaced the pump yesterday. The car doesnt over heat before or after the pump replacement...goes up to 100 fan kicks in, and down to 80.

It still likes to spit out coolant from the overflow tube to the point it gets low and the dash light comes on. The head gasket was replaced twice last year according to previous mechanic bills. And no I dont trust the quality of their work as they reinstalled the wrong type of spark plug wires and left a couple bolts loose. Remanfactured head installed by previous mechanic last May.

1987 2.3 16V 177,000 miles. On the plus side it pulls like crazy and idles at 1,000 when at running temp. I do have an issue when the ICV is plugged in....it stalls.

Am thinking possibly the radiator (pressure) cap, the cap on the tank in other words.

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Crap! Sorry, I forgot to mention the cap is new as thats the first thing I thought of also.

lear, a new cap could be faulty, especially if some aftermarket brand. Take it you replaced the thermostat when you replaced the water pump? Best to buy a reconditioned water pump from your local MB dealer ... 1 year parts warranty.

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Fred Hoelzle

Ok, I just got home and will perform a compression test just for the hell of it. While Im at it, Ill use the compressor to pressurise the cylinders to see if it bubbles back into the expansion tank.

The thermostat looks new as I inspected it when the upper rad pipe was off. Didnt think a bad thermostat could keep the system pressurized.....

Edit: 180-181 PSI across the board. and cheepo radiator cap. Ill go to MB in the morning and get a new cap and thermostat.

lear, a properly functioning thermostat should maintain an even engine operating temperature unless the ambient temperature is extremely high and you are sitting in traffic. The expansion tank cap controls the cooling system operating pressure, presuming the cap is functioning properly.

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Fred Hoelzle

Thanks Ferdman. While I was testing compression, I attached my compressor to the engine compression line and ran it up to 120psi. While attached I hand cranked the engine over to see if any bubbles were coming into the expansion tank. Nothing on all four cyliders.

The new cap and thermostat are on. Im waiting on a new belt tensioner assembly to get here hopefully tomorrow. So I took the opportunity to remove the valve cover to check the timing marks on the cam sprockets. Lined up perfectly TDC. (Im having an idle issue and a coolant loss issue).

Ill put everything back together when the parts get here and well see.

Thanks for the input. At least I know I dont have to pull the head and I have good, even compression across the board.

lear, please explain the term "engine compression line" because I have never heard that term before. I guess you realize that to test the compression on each cylinder you would need to remove all the spark plugs and test each cylinder separately by turning the crankshaft manually with a test gauge inserted into the spark plug hole.

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Fred Hoelzle

Late response, I was out of town.

I had heard that one way to check for a blown and or leaky head gasket, is to put compressed air into the cylinder and hand crank the engine a full cycle. If you get bubbles, from the compressed air, into the expansion tank....you have problems.

So I hooked up the compression tester to cylinder one. Instead of attaching the pressure gauge, I attached a line from my shop air compressor and pressurized to 120psi. I then hand cranked the engine to check if air was escaping from the cylinder into the coolant system. I repeated the process on cylinder 2,3 and 4.

Hope that is a better explanation.

Moving forward, the new expansion tank cap and thermostat is installed along with a new belt tensioner (old one was shot). Today is play with the car again day so fingers crossed. Ill update appropriately.

But wait, there's more.

When using a cylinder compression hose as a cylinder leak down tester, you must remove the check valve at the spark plug side of the hose. The valve is usually a tire valve core.

Also, you will need to rotate the engine so the cylinder under test has the piston at top with both valves closed before you apply air. When doing this, apply a bit of air at first to make sure the motor does not roll over. ( Keep clear of belts and fan. )

The engine will roll over with some force when not at TDC and air applied. So if you were able to roll the motor over by hand, no air was entering the cylinder.

Really rough calc below:

3.5" bore at 100 PSI = 962 LB force ( PI * R^2 = area^2. area * PSI = LB force )

3" stroke = 1.5" lever with 962 LB hanging on it = 120 Ft/ LB ( with the crank at 90* )

97 SL320 how dare you interject math into my dummy thread hahah.

Your right....I just went out to the garage and Ill be darned.....schrader valve. Leaned something today, thanks. So school me this, #1 cylinder can be verified TDC with valves closed by looking though the oil cap. How do you determine valve position on the other cylinders? Perform test with the valve cover off?

Bright side of the story, no more leak. Well there was one coming from the heater control valve which I knew was bad and a new one will be here early next week. I used a piece of double ended barbed nylon splice tubing to test the pressure. Took it up to 100c to test the thermostat / fan etc. No leaks coming out of the overflow tube.

You can do it with the cover off if you want, or if it's already off anyways. Otherwise just go to each plug hole and rotate the engine through and watch for the piston to come up to tdc (following the firing order sequence). If there is a cyl you can't see you can use something like a wooden dowel in the hole and watch that to come up as high as you think it will go. You can make a gauge mark on it on #1 when you do that if you are afraid of going past tdc.

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Remove the rad cap, pull all the plugs, insert the compression tester hose ( sans valve ) into a cylinder. Rotate the engine by hand ( either by crank pulley or a socket on the alternator pulley ) , listen / feel for air coming out of the hose _while you are turning_. When the air stops the piston will be very close to TDC / both valves closed.

Roll the motor back and forth, feel for a point where there is little resistance to turning, center this point. This low resistance is due to crank turning but pistons not moving.

Mark the crank pulley if you like, be sure to remove any tools from the crank / alternator. Hold the quick connect sleeve on the air hose back, do a quick plug / unplug so a puff of air enters the cylinder. If the engine does not turn, rehook the air then look for bubbles in the rad cap. Leave it attached for a minute or so.

If the motor rolls over, turn and try again. The trick is a short puff of air so the motor does not roll over too far so you can find TDC rapidly.

You _will_ get air leaking out of other areas, don't be alarmed. There will be hissing in the oil cap ( rings ) out the tail pipe ( exhaust valves ) , sometimes air filter ( intake valves ) and if the head gasket failed between cylinders, the next spark plug hole ( OK, now you can be alarmed, these are listed in order of engine wear.) .

The exhaust sensing fluid that you put over the rad cap won't find small leaks, a leakdown with air is the only way to test engine health.