Gradual overheating while driving - Page 3 - PeachParts Mercedes-Benz Forum

JimFreeh · #31 05-23-2007, 08:00 AM

I learned this lesson the hard way.......

There are two things you need to look for when testing a t-stat.

First, is to observe the temp it opens at.

Second is to measure the stroke of the t-stat.

MB cooling systems require the t-stat to backseat in the housing to eliminate the hot head water from short circuiting the radiator and going directly to the water pump.

If this stroke is insufficient to back seat the t-stat, you will have similar symptoms.

I'm not familiar with the iron block 4.5 engine, but on my OM61X and OM60X diesels, I know this is the way the coolant sytem works.

Jim

michaeld · #32 05-25-2007, 12:15 AM

Hi, Jim,

I'm not sure what you mean by "measure the stroke" of a thermostat. No idea, in fact. I guess I'm also not positive what you mean by "backseat."

I didn't just check the thermostat, I replaced it. And I used the new seal ring that was included. I had the Mercedes manual and attempted to follow the procedure exactly as described. I don't recall the manual saying anything about measuring strokes or backseating, or using any kind of sealer on the thermostat.

250 Coupe · #33 05-25-2007, 09:16 PM

When a valve is designed to "backseat" it has two sealing surfaces, one at each end of it's stroke.

Imagine a capital "T". With the water coming in at the base, you want it to go left to the radiator or right to recirculate back to the engine.

Most thermostats just control the amount sent to the radiator and let a small bypass hose do the recirculation. MB controls the temp by using a valve that has two seats.

Somewhere along the line, I posted a link to a diagram of the cooling circuit that shows how the thermostat works but I don't remember when or where. If you'd like, I'll try to hunt it down again.

Michael

michaeld · #34 05-26-2007, 12:13 AM

250coupe,
Thanks for the explanation. And yeah, if you could relocate it, I'd really appreciate a chance to obtain a better understanding of how the Mercedes system works. I frankly don't quite get the thermostat in the LOWER hose thing.

250 Coupe · #35 05-26-2007, 11:21 AM

I found it!

http://handbook.w116.org/Engine/107/m117_45/20-005.pdf

Michael

michaeld · #36 05-27-2007, 02:06 AM

Thanks 250coupe,
I saved it to my computer. I've got another diagram (also from the 116 handbook) somewhere, and would like to compare the two together.

As I've said, this has been a very difficult diagnosis. Ordinarily, when I have difficulties with diagnosis, I take the car to a mechanic and have him do it (I am pretty good with taking a part off a car and then installing an identical part the same way; what I lack - which separates me from a good mechanic - is the ability to diagnose problems (beyond reading the 'troubleshooting sections' of manuals)).

But I've now taken the car to several mechanics (including a German specialist), and gotten nothing from them. Given the fact that my car has actually seemed to run pretty well (well, until it overheated), I never would have thought of my fuel mixture if I hadn't seen that 0.00 %Co on the smog test.

I'm hopeful that my issue will turn out to be a fuel mixture problem. It will take me a couple of weeks to arrange to get the car in, but I won't mess with anything else until I've addressed that. I'm grateful for you guys for all the help.

Tomguy · #37 05-27-2007, 03:19 PM

Have you thought about making that adjustment yourself - the 1/4 turn on the mixture? Something that will help "Undo" it is to mark the current position with paint so you can recall where it was, if need be, for next year's emissions test. You'll almost assuredly notice more power, too.

michaeld · #38 05-28-2007, 12:31 AM

Quote:

Originally Posted by Tomguy

Have you thought about making that adjustment yourself - the 1/4 turn on the mixture? Something that will help "Undo" it is to mark the current position with paint so you can recall where it was, if need be, for next year's emissions test. You'll almost assuredly notice more power, too.

Hi Tomguy,

Yeah, I have - after you gave me a figure (the 1/4 turn). W/o that, I'd have NO idea how much turn to give it.

Richening the mixture will help give me a better idea if THAT'S the cause of my problems. I'm really hopeful that it is.

Particularly if it seems to help, I figure I'll probably want to get the system "zeroed in" by pros with an exhaust analyzer who can precisely set the mixture for me.

As a historical note:

I've had a bad brake booster for some time (the brakes work fine, just no power assist); some time back a friend suggested that maybe the vacuum leak from the bad booster was causing the lean mixture. I disconnected the vac line and plugged as a test, but nothing changed.

I didn't want to replace the booster until I had fixed the overheating situation, as there was only so much money I intended to invest in the car. But if I get the overheating issue under control, I'll turn to the booster. I'm wondering if the bad booster could have had anything to do with the fuel mixture getting out of whack.

Tomguy · #39 05-28-2007, 04:13 PM

If anything, the leak would make you run richer I'd imagine... that's how it is on any FI engine I can think of, but those are all EFI with a manifold pressure sensor. And if it's a bad leak, it'd be a lot richer.

Have you ever checked the color of your plugs? Probably white...

250 Coupe · #40 05-28-2007, 09:35 PM

The air/fuel ratio is controlled by the air plate and pin in the fuel distributor. Any air sneaking in after this point is unaccounted for and will make the engine run lean.

Michael

michaeld · #41 05-29-2007, 10:11 AM

I attempted to answer the question, once for all, whether the leaking booster could be the source of my problem.

I disconnected the vacuum hose and plugged it off so it was airtight. In other words, I temporarily eliminated a vac leak from the booster from the equation.

When I test drove, the car didn't take either longer or sooner to overheat. And the brakes weren't any different either!

It sounds like it was at least conceivably possible that a leak from the brake booster vacuum line could have initiated a chain reaction over time that could have gradually affected the fuel mixture.

If getting the fuel mixture corrected resolves the overheating issue, I'll replace that booster pronto.

meltedpanda · #42 05-29-2007, 11:12 AM

Did you make sure the little arrow on the thermostat is in the correct posistion ?
What kind of plugs and what octane are you running?

250 Coupe · #43 05-29-2007, 08:14 PM

Michael,
Was there vacuum at the hose you pulled off the booster? If your booster is leaking, the engine would run noticably different with the hose removed and plugged.
Two things come to mind. One is that the hose is installed backwards so that the check valve is preventing air flow out of the booster, the other is that the check valve is stuck shut.

Michael

michaeld · #44 05-30-2007, 02:24 AM

Quote:

Originally Posted by 250 Coupe

Michael,
Was there vacuum at the hose you pulled off the booster? If your booster is leaking, the engine would run noticably different with the hose removed and plugged.
Two things come to mind. One is that the hose is installed backwards so that the check valve is preventing air flow out of the booster, the other is that the check valve is stuck shut.

RE: my thermostat, please refer to my previous posts. If I replace my water pump, I promise that I will replace the thermostat (again) as a matter of routine. Otherwise, I have NO REASON not to believe it's working fine.

250Coupe,
When I asked Brian, the owner of MotorWerks, if a bad booster could cause a lean mixture - and whether disconnecting and plugging the vacuum line would be a good test to rule this out - he pooh-poohed the whole idea and stated definitively that the engine would not even run with the booster vac line plugged. He was obviously totally and completely wrong, given the fact that I drove the car around with the line plugged off with no "issues" other than the overheat problem I'd already experienced.

Now, Brian has a reputation for being a top notch German auto mechanic. He's THE guy you take your Mercedes to in Palm Springs. But he was clearly going on some kind of theory that wasn't actually right.

That said, I am a little bit on the skeptical side now when I read what you're saying (recognizing that you might be a qualified, top notch mechanic who turns out to have a misunderstanding of the booster vacuum line system just like Brian did). Or you might be right (given that Brian's assessment was wrong). I would be suprised if the vac line were installed backward (it's been connected as is since I've owned the car), but you could be correct about the check valve being stuck shut.

First of all, is this check valve in the hose itself (in the thingy that looks rather like a filter)? Or is it on/in the engine connection? Second of all, could a stuck check valve cause an overheat situation?

As I said, I could not tell any difference plugging off the vacuum line or driving with it connected to my bad booster. If someone with a healthy cooling system in a M117 wanted to test whether disconnecting and plugging the vac hose significantly altered the engine performance (particularly if it caused the engine temp to increase), I would be very interested in the results.

BTW, Tomguy mentioned checking my plugs. When I pulled out the set that had been in the car for near two years, they basically looked fine. The ceramic insulators were in good condition, and the electrodes - and even plug gaps - also appeared to be fine. Mind you, I'm no expert at "reading" plugs (I have to take out my Hayne's/Chilton's manuals and compare the pics to the plugs, but they appeared to be in the normal range.

250 Coupe · #45 05-30-2007, 02:44 AM

With the engine running normally and your foot off the brake, the booster can is at manifold pressure (yes, I mean pressure. Anything above absolute zero is pressure).

When you press the pedal down, the vacuum helps by pulling the diaphragm. When you hold the pedal down, the vacuum builds back up to manifold pressure. In effect, each press of the pedal uses a chunk of air rather than a continuous flow.

The check valve is the filter looking thing in the hose. It keeps the engine from lowering the vacuum in the booster when you accelerate. It also will hold enough vacuum in the can for a couple of assisted stops if the engine dies while you are driving.

The idle mixture is calibrated to whatever very small leaks are present in the hose/booster combo. Disconnecting the hose with the engine running will kill it instantly. Rapidly pressing and releasing the pedal will head in the same direction but, with a lesser loss of vacuum, the idle will just stumble a bit. The engine will run fine with the hose disconnected and plugged but may end up being a bit rich due to less air sneaking past the air plate in the fuel distributor.

Michael