Could a brake booster vacuum leak cause an overheat?

[michaeld]

Guys,

I'm STILL a little concerned about running too hot on the road!  Yes, yes, I know, it IS pathetic, isn't it?

I took the car to MoterWerks (which is a Mercedes specialist, as the name might suggest), and they ran the car at idle for 2 1/2 hours with no problems (in fact, it ran cooler than the owner would have figured for such an old car).  He drove it briefly, but wasn't able to find any problems.  They pressure tested the system, and that sort of thing.

He said the rad looked great (it is a pretty new recore), the water pump was pumping, the fan clutch was clutching, the aux fan was auxing, etc.

That's three mechanics (four, counting a personal friend who retired as a tranny shop owner) who've said the car was fine - yet I'm still showing hot after long drives (and 2 gauges/sending units wouldn't lie!).

So, here's my question:

Could a vacuum-leaking brake booster cause a too-lean fuel mixture in the engine and thus a "run-hotter-than-you-wanna" situation?  When I push the brake pedal, I can actually HEAR the vacuum escape.  I haven't wanted to mess with my brakes until I dealt with the overheat, but could the one be causing the other?

Another possibility is FAR more frightening: a flow-issue in the cylinder head.  I wouldn't even know WHAT to do about that!

Now, I suppose it's POSSIBLE that my new radiator is blocked, or that the fan clutch or water pump is bad; but would four mechanics miss that?

One other question: if I WERE running too lean on the fuel mixture, would that help me pass my smog?  I've got to do that soon.  I'd HATE to go to the bother to fix one problem at the expense of another!

[koan]

Could a vacuum-leaking brake booster cause a too-lean fuel mixture in the engine and thus a "run-hotter-than-you-wanna" situation?  When I push the brake pedal, I can actually HEAR the vacuum escape.  I haven't wanted to mess with my brakes until I dealt with the overheat, but could the one be causing the other?

I would think it very unlikely that a leaking brake booster could supply sufficient air to lean out the engine and still be a functioning brake booster.

The sound you hear is the atmosphere pressure replacing the vacuum one side of the booster diaphragm, this is where the boost comes from. Unusual to be able to hear it though. If the sound is there constantly with your foot on the pedal at a a fixed position there is something wrong.

The pedal will sink a bit if you have your foot on and start the engine.

One thing about M-B cooling systems that is different from others I've worked on is that there is no wound "spring" in the bottom radiator hose. The bottom hose is on the suction side of the water pump and can collapse without the spring in there to hold it open.  The spring can disintegrate in a neglected cooling system and causes the exact symptoms you describe. OK at idle (low suction) but cruising (high suction) collapses the hose and restricts water flow.

M-Bs don't have this spring so it can't be your problem but next time the car is on a hoist it would be easy to give it a few thousands revs and have someone look at the bottom hose just to make sure nothing odd is happening,

koan

[torana68]

you would know (probably) that the booster was leaking badly enough to cause any other issues, for example the car most likely wouldn't idle, and your brakes wouldn't be much chop. You should have 3 applications with vac before you run out of air( actually run out of vac),  run the car, turn it off three pushes you SHOULD hear the air noise, after that the pedal will get harder then hard and no noise. What do you call overheating? all cars go through stages, cold, warm then operating temp which isn't one set point, everything is busy absorbing heat until it cant absorb more, thats when the gauge starts going up again, if the radiator is good it will stabilise in relation to load, more load more heat, higher temp, BUT you should be able to run the engine at max load/revs without it overheating (some cars can some can't I had a Torana that would overheat at a constant 180+K's, you had to slow down for 20 min or so before speeding back up for about One hour at high speed). You may have a lean spot in the fuel injection ? dud fuel pump etc? check the spark plug colour?
Roger

[michaeld]

I get the feeling you guys don't think my brake booster is related to my running hot issue.

Koan says, "I would think it very unlikely that a leaking brake booster could supply sufficient air to lean out the engine and still be a functioning brake booster."  He's probably right about that; but I don't think I HAVE a functioning brake booster.  I believe my brakes are basically operating "booster free," i.e. like manual brakes.  Is the booster STILL ruled out for creating vacuum woes that could result in running too lean, and thus too hot?

I'll be replacing my spark plugs in the next couple of days, and am interested what they will look like.

I've never looked at my lower hose while someone revved it, but I replaced the hose when I replaced the thermostat after I first started running hot (with a quality OEM hose).

To be honest, I'm not quite sure what torana68 is getting at.  In terms of his question, "What do you call overheating," I'm not technically "overheating," in the sense of boiling over and that sort of thing.  But I'm gradually just running hotter than the engine ought to be running.  I'm told that old Benzes will run 210F, even up to 220, but I'm climbing above that slowly but surely as I drive - and the car didn't used to do that (used to run under 195, even down to the low 180s).

The car never gets hot at idle (even after 2 1/2 hours, I now know!).  But as the system is under load (driving) it gradually gets hot.

Back to Torana's last questions: what would a "lean spot" in the f.i. look like?  If I had a dud fuel pump, wouldn't I stall out and not be able to restart (the car starts and runs fine).  And yeah, tomorrow or the day after I'll check the plugs' color.

[torana68]

ok... are you loosing any water? if the EFI is dodgey (for example low pressure) you could go lean and cause heat as can .... sit down.... a blown head gasket
Roger

[Tomi]

hello Micheal,

Your hot motor when running and not when idling could well be a lean mixture condition. This could happen if you are not getting enough fuel at high rpm. and I think the following could result in these conditions:

- a clogged fuel filter, which gives enough fuel only for idle conditions.

- how about the air measuring plate under the air filter. is it clean, if not clean the edges and see that it is well centered. when you press it by hand, does it move without any frictions. If it gets friction or binds, the main piston inside the fuel distributor is clogging (and theres almost nothing DYI can do except run 2-stroke gas (some oil 2 stroke added) and see if it gets better.

- there could be other issues like leaking vacuum lines (particulary the ones going to the ignition distributor)

I agee with the other guys, that the first think to do is after a hard run when the temp has been high, stop the car, wait it to cool and check one plug for the color. If you are on lean conditions it may be dangerous, check the above and have the k-jet adjusted (mixture, fuel delivery measurement etc.) by a specialist(its not expensive).

yes, head gasket is a possibility

(our MB club organised a full day course on the k-jet, where some old pro's explained the system and gave some good practices)

regards, Tomi

[koan]

To verify the brake booster operation you could disconnect the vacuum line at the the booster, put a bung in the end of the line or tape it up closed, then go for a drive. Go very cautiously as it requires a lot of pedal pressure to stop a 116 without boost.

koan 

[WGB]

It might sound a bit basic but these motor's are very sensitive to spark advance - particularly if too far advanced.

I would check that the timing is exactly as per specs and not where someone with a dyno thought it ran best.

They will run hot if advanced.

Another check is the fan coupling if this hasn't been done before - easy to check, just slowly increase the revs with the bonnet open and listen for the fan roar which should suddenly stop at about 3,500 revs as the fan de-couples. A problem with this will cause the car to run hot in traffic but slowly settle when the speed increases.

Also check that although the radiator has been recored that the motor may not have been flushed and subsequently has been gunked up again. ?Have you checked your thermostat.

Bill

[michaeld]

Some good pointers; I'll cut and paste them into a file to read them better and have them later.

Dealing with the "probably aint that ones" first;
- Thermostat was replaced, and the old unit passed the "boiling water" test. 
- I've never had a scintilla of evidence of a head gasket issue: no white smoke (I mean none), coolant and oil look great, no gunk in the cap, and passed pressure tests.  All three mechs said the head gasket was fine (after I brought it up).
- I'm not loosing any coolant; once I drain the system, I have to add coolant several times after the thermostat opens, but then the level is fine after that.  The thermostat clearly opens at the specified temp (175F).
- The fan clutch is working; it does change pitch when the engine gets warm; it passes the "rag test" (spins forcefully rather than freely), and I trust the mechs would have caught this.
- I had the system backflushed several months back and still had the "issue"; I just went ahead and changed the coolant - and flushed the system - first.
- The last time I checked the timing it was quite close to spec, and that was after the heat issue began.  I'll check it agan, just to make sure.

Koan,
That was my plan (plug the booster vac line and drive).  I was wondering how strong the vacuum was, and whether something like tape would do.  I actually think I've been braking w/o a booster for a while now.  I am a big guy, and I used to be a weightlifter, so I've been 'handling' it.  I'll see how much more "boost" my defective booster was giving when I block it off.  I was somewhat surprised when the MotorWerks owner said "The engine won't run if you do that."



Now, for the "Well, that could be my problem" category:
- A fuel issue was mentioned quite a few times.  I am a flat-out ignoramus when it comes to EFI isses.  I'll probably take it to someone who knows what the heck they are doing unless someone can offer something specific to check that doesn't involve having an "exhaust sniffer."
- I have a new fuel filter and was planning to change it.  I guess that ought to be my first priority (before taking it to have the EFI dealt with).

Tomi added a couple of possibilities:
"- how about the air measuring plate under the air filter. is it clean, if not clean the edges and see that it is well centered. when you press it by hand, does it move without any frictions. If it gets friction or binds, the main piston inside the fuel distributor is clogging (and theres almost nothing DYI can do except run 2-stroke gas (some oil 2 stroke added) and see if it gets better.
- there could be other issues like leaking vacuum lines (particulary the ones going to the ignition distributor)"

Could I check those distributor-related vac lines with a stethoscope at idle?  I have no idea how much vac they should draw.
I'll look at that "air measuring plate" thingy; I THINK I know what you mean.  The last time I checked it, it didn't seem to bind at all.  I'll be changing my air filter, and that will be the time to see.  I'm not sure what you mean by "running 2 stroke gas"; do you mean in the fuel tank, or squirting it into the plate?

Last question:
If I'm running lean, would that help me or hurt me with passing smog?  If it would help (and I can't believe you'd want to RICHEN your mixture to pass smog) I might want to wait until after I've smog-tested?

[koan]

A few layers of tape over the end of the pipe and a few turns around them and the hose should do it.

Leaning out the engine reduces CO and HC emissions but increases oxides of Nitrogen because of the higher combustion temperature, what do they measure at a smog test ?

My suggestion would be to do the easy stuff first, the hard, expensive things like head gaskets should be last on the list.

ken

[WGB]

This was mentioned earlier on another thread - am I correct?

I think we talked about actually checking that the gauge was correct - I assume this was done.

I don't think M-B's in general suffer bleeding problems when the coolant is replaced and I guess the heater core has been bled and is capable of putting out hot air.

Bottom radiator hose collapsing when suction applied with revving the engine but OK on idle?

Just some simple things that jumped to mind.

Bill

[michaeld]

I actually installed a new aftermarket gauge that seems to be reading accurately (eg. the thermostat opens when the gauge shows 175-180F, and the gauge reads in the 212F range when the aux fan kicks in just like God intended.

The heater puts out hot air.

I suppose I can crawl under and inspect the hose while someone revs it; but - given the hose is in new condition and replacing the former hose did not "fix" my problem - the collapsing hose would itself indicate some OTHER problem.  I know the hose does not collapse at idle even when fully warmed-up.

I'm burping the system properly, installing the coolant on an uphill slope, and then running the engine until the thermostat opens.

I'll check the timing w/in the next day or so, when I inspect and replace the plugs.

I found Koan's last post re: overly lean mixtures to be correct when I checked; it DOES increase the NOx (nitrogen oxide) measurement.  CA tests 1977 models for hydrocardons (HC), carbon monoxide (CO), and NO (Nitrogen oxide).  I can fix the first two running lean, but at the expense of the latter.

The car starts up, runs, accelerates, and idles fine, and always has.  If I had a fuel injection issue significant enough to cause hot running, would there be any sign of it (beyond the hot running)?

Could a bad catalytic converter cause the car to run hot?

Does my 77 Federal emissions 450SEL have an O2 sensor (I don't remember seeing one on the exhaust)?

[koan]

I'm burping the system properly, installing the coolant on an uphill slope, and then running the engine until the thermostat opens.

I can't get the cooling system full on my 6.9 engine when the car is on ramps, don't know about 4.5s.

Could a bad catalytic converter cause the car to run hot?

If bad means blocked I'd say yes, a significant restriction anywhere in the exhaust system could require more throttle and therefore more heat to cruise. Any abnormal condition like dragging brakes that needs to be compensated for by more power will make more heat.

koan

[michaeld]

Ah,
Here's my patented method of burping.

First I add coolant to the full mark on the reservoir.  Then I push the car out onto my fairly steeply sloped driveway so that the front end is up.  Then I wait for the coolant to get sucked down as the thermostat opens and begin to add more coolant until I have it filled to the proper level again.

The slope is enough to make the radiator the high point, but not so high that I'm not able to accurately read the level.

I have found that after driving the car around some, I often need to slightly top if off once or twice.  Then it's good to go for the long haul.

If a bad (by which I mean blocked) catalytic converter could be the cause of a gradual overheat, I'll have to put it on my "Maybe it's THAT" list.  I don't think I've got a dragging brake issue, as I'd undoubtedly catch brake drag doing my "burping" exercise (see above!).

[WGB]

I had a 126 300SE that defied all attempts by the agent to keep cool in the early 1990's (original problem was probably a fan hub which had lost it's stick).

In the end they put in a lower temp auxillary fan switch so that the fan stayed on a lot of the time - it fixed the problem (After two plastic radiator tanks split and the radiator was re-cored)

There is also two grades of thermostat - one opens at 80 degreesC and one at 85 - the 85 is the later one and certainly in the 126.

I subsequently learned in the last 2 or 3 years (from Mercedes Enthusiast Magazine) that in the 90's the amber coolant that was used allowed some aluminium corrosion which slowly silted up the radiators.

This was rectified by the change to the newer Blue coolant.

Bill