Odd Fuel release during a compression test?? - PeachParts Mercedes-Benz Forum

dog3leggeddog · #1 01-13-2017, 04:55 PM

Hello All,
I am new to the forum and have a question about what possibly may have gone wrong during a compression test. My car is a 1981 240D automatic W123. I previously owned the car and recently purchased it back from the individual that I sold it to. The car was running, starting fine when I sold the car a few months ago. It was a little cranky on cold mornings, but would start. I notice the car on craigslist again a few months later stating that it needed a new engine. Long story short, I bought the car back to part out and keep a few things. Just out of curiosity I thought I would see why it would not start. Checked valve adjustments and cam timing all OK. Pulled the injectors to do a compression test with less than happy numbers. #1-300,#2-150, #3-200, #4-255. The weird thing is when I did the last cylinder the engine started gushing diesel out of the injector hole #1. I'm not sure if I did something wrong here. I put plastic caps over the injection pump fittings and held the stop lever to halt fuel to the pump. My guess is that fuel pressure was building up in the pump and dumped fuel into the engine from the back side of the pump. SO, is this possible and do you think my compression numbers are even accurate? I would not think the engine would have ever ran with numbers that low and I was driving it months ago. Any thoughts are greatly appreciated.
Regards.

SD Blue · #2 01-13-2017, 08:26 PM

I'm trying to understand what you are describing. Is it possible the #1 injector is bleeding fuel at very low pressure or stuck open due to damage?

dog3leggeddog · #3 01-14-2017, 01:19 AM

No all of the injectors are out and fuel lines removed. I have never seen this before. I have no idea were the fuel is coming from. I guess my biggest concern is the low compression numbers. If this motor is this worn, I am really surprised that it ran at all and with decent power at that. Kind of weird I think.

Diseasel300 · #4 01-14-2017, 10:41 AM

So to clarify, you had the hard lines removed from the injection pump and the fittings on said injection pump capped with plastic. Injectors removed from engine, correct?

The fuel that was coming out was coming from the prechamber hole where the injector screws into on Cylinder #1? Are you sure it was fuel and not coolant?

SD Blue · #5 01-14-2017, 11:38 AM

What does your oil level look like? My thinking is fuel sitting on top of the oil.

Does your engine have the diaphragm-style of fuel pump? If so, I've heard of the problem of fuel getting into the oil when the diaphragm is compromised by a pin hole.

dog3leggeddog · #6 01-14-2017, 01:01 PM

I did check the oil level and it was within the acceptable range and did not smell of diesel. I am wondering if you are right about the coolant though. I may be seeing two unrelated things here. Yes the the pump is capped, lines removed, all injectors out. The clean diesel on the floor may be directly from the pump and the fluid coming out of the injector hole is something else perhaps coolant. With that said, how did any liquid other than oil find its way into the cylinder? Remember that this was the best cylinder of all at 300 psi. Even if the pump is leaking into the engine how would it make its way in to cylinder #1? Is it possible that the head gasket has failed? If so, is it possible to have failed across all 4 cylinder? Perhaps my next step is to perform a leak down test to see were the cylinder is compromised. I'm not sure which type of injector I have. How do you tell? Was my precautionary attempts to cap the injector pump a mistake?

kerry · #7 01-14-2017, 02:46 PM

I saw that car for sale and wondered if the owner was wrong about it needing a new engine. Something is odd there. Did the owner run it out of oil or something?
I have compression tested thru the glow plug holes and left the injection system intact, just pulling a vacuum on the shut off valve while doing the test. Worked fine.

I have no idea how diesel came out of #1. Can you get it to repeat? As I recall, the car had about 220k miles on it. Is that right. Original engine? It might run with compression numbers that low in very warm weather. I assume the owner couldn't get it to start in the fall sometime. Did it run fine in cold weather when you owned it and how many miles/years ago was that? What's the coolant level like?

Diseasel300 · #8 01-14-2017, 02:51 PM

There shouldn't be any fluid in the cylinders. If you have something squirting out of the cylinder, you likely have a head gasket leak which can also explain the woefully low compression numbers.

dog3leggeddog · #9 01-14-2017, 05:41 PM

I sold the car early this fall. We had already had temperatures in the 30's so I think it probably would had started fine with a block heater in colder temperatures. He owned the car for probably a month and a half. The kids that I originally bought the car from had just drive to Vermont and back to Steamboat, CO.. I thought if it had enough power to traverse Rabbit Ears pass a few times it should do fine around town. I owned it for a few months myself and it ran fine with plenty of power, never ran hot. Long story short, I would love to get the car running again and enjoy it, but I am not out much if not. So, I agree that the only thing that could be in the cylinder is coolant. The coolant level is a little low (1/4" above the core looking down through the top tank). Could just be coincidence that the head gasket finally failed while I was doing the compression test. I think the most logical step would be to do a leak down test or just pull the head. If the motor truly needs a rebuild it won't be worth much more than parts.

kerry · #10 01-14-2017, 06:14 PM

If there was enough coolant getting into the cylinder to make it visible, wouldn't the engine have hydrolocked? You could try pressurizing the coolant system by borrowing the tool from an auto parts store and see if you can see or hear coolant entering the cylinder.

97 SL320 · #11 01-14-2017, 07:31 PM

Better than pressurizing the cooling system is to pressurize each cylinder with both valves closed. This is the best way to test for a combustion chamber to coolant / intake / exhaust. It also checks for piston ring leakage.

kerry · #12 01-14-2017, 07:33 PM

How do you know if the a dropping pressure is going into the coolant passages?

dog3leggeddog · #13 01-14-2017, 08:39 PM

I thought about the coolant pressure test because it would be the quickest, but it wont tell me if there is a leak in the valves or rings. Also, I would think if there is a failure with the head gasket you would see bubbles in the radiator or a release in the adjoining cylinder.

kerry · #14 01-14-2017, 08:47 PM

The cooling system are notorious for being difficult to get air out of them hence my question and suspicion the air might not immediately migrate to the radiator.

97 SL320 · #15 01-15-2017, 07:31 AM

A combustion to coolant air leak using the cylinder pressurizing method will make it's way to the rad fill point as bubbles or for a very slight leakage, the liquid level will rise at the rad fill point even if there is an air pocket somewhere. Just make sure to fill the cooling system to the top of the fill point and the cap is off.

The CO method of checking for a combustion to coolant leak is quick however if the leak is small it takes a while for the color to change. If the leak is large, bubbles will come from the cooling system more or less making the CO test redundant.

Head gaskets can fail in numerous ways, cylinder to cylinder with no coolant loss , cylinder to coolant , coolant to oil , coolant to outside of engine. Being generic and saying a head gasket failed for all engine faults is like saying the electrical system "has a short" when there really is an open circuit due to a broken wire.

I've been in and around the car business for 40+ years and have diagnosed / built countless engines, pressurizing each cylinder is the only way to test for this kind of leak.