Vacuum leaks

[BillKerler]

I have always had the problem of my car idling when up to temperature at about 725 rpm in neutral, and when put into drive the rpm falls to 575 rpm. A 150 rpm drop always seemed too much. I found a vacuum leak which I suspect many other '94 C280's might have.
On the passenger side under the hood close to the firewall is a black plastic cover. One of the items under this cover is a block manifold for several vacuum lines to feed off of (it's next to the diagnostic plug in port). This manifold develops a crack...it's not a question of will it crack, but rather when will it crack. What looks like a little ball bearing at the end of the manifold, look on the back side of the manifold.
My crack was about 3/4"long and I could get my fingernail in it. Rather than replace the block manifold and possibly break other vacuum tubes connected to it, I repaired it. I thoroughly cleaned it with a clean rag & lacquer thinner. Then I used Loctite high temp extreme epoxy and layed down a coat over the crack. It takes 24 hours to cure hard as a rock.
What a difference. My car now idles at 600 rpm in neutral at temp and when I slip it into drive, the rpm only drops to 550.
One other note, I no longer get P1700 CEL...when going 26 to 28 mph under 55% load the rpm should be below 2100 (ie, you should be in 3rd gear). If 3 times in a row the rpm is too high (because you are still in 2nd gear) then the CEL will come on with P1700. Plugging the vacuum leak now has me starting off every time from standstill in 1st, by 10 mph I am in 2nd and by 20 mph I am in 3rd. I shift into 4th about 33 mph.
If you don't come to a complete stand still when coming to a partial stop, the car will start up again in 2nd. A complete stop (more than 1 second) will get you to start in 1st...even with a very gentle foot on the accelerator.
One last thing, check the white vacuum line under the black cover on the front of your engine...this line goes to EGR equipment. The engine is so hot, this line becomes very brittle. Replace it.

[97C28O]

Could you post a picture of the where all this is located? :rolleyes:

[BillKerler]

Go to this thread
http://www.club202.com/forums/showthread.php?s=&threadid=987&highlight=woot+just+finished
About half way through his write up is a photo of the area near the firewall on the passenger side. He has the black plastic lid (about a 15" by 15" piece) off. See the round electrical connector for diagnostic by a Mercedes shop? Next to the port is the manifold block with the vacuum hoses. The ball bearing is on the end of the manifold closest to the engine. Look on the back side near this ball bearing for a crack in the plastic.

[Sulaco]

Bill,

I'm VERY glad to see someone with an m104 w202 being so thorough and (most importantly) active on all these issues. You and I have the exact same car, so I find all of your information very useless. Per your thread on o2 sensors and fuel consumption, I've ordered a pre-cat o2 sensor to replace my stock one (235k miles on it!) to try and fix my noticably-worsened mpg.

But back on topic with vacuum leaks; I KNOW I've got some serious vacuum leaks. About 5-7 years ago I replaced that block under the hood with all the vacuum lines on it. It might even be worn out again (I haven't looked yet.) But I know i've got other issues and my transmission DEFINITELY behaves weirdly.

But it's had odd behavior for years.

I had it rebuilt somewhere below 100k. It worked flawlessly until about a couple months later (just outside shop warranty) it began to do ONE thing really weird: when hitting the gas to shift down from 4th to 3rd, it would downshift EXTREMELY hard. Other than that, it behaved normally. Outside their pathetic warranty, I couldn't do anything about it. So I just avoided downshifting like that. I'd either floor it so the kickdown would hit 2nd, or feather it and avoid downshifting. That problem eventually, uhh, softened. Stupid of me. But I digress.

For the last 2+ years it's had a widening gap between third and fourth--ESPECIALLY when cold. In the wintertime, before things are up to temp, it will shift from 3rd to nothing. I have to rev it up quite a bit (like 4k) several times before it catches 4th gear. As the trans warms up, it behaves more normally (though not "normal" if that makes more sense). Lately I've noticed a slight hissing/whistling/sucking sound when my foot isn't on the gas.

THe gap between 3rd and 4th is always there, it just lessens with higher ambient/engine temps.

Never done a flush/change on the trans. I've heard nothing but horror stories on these things post-flush. Is that some kind of urban legend?

Where would you begin?

[BillKerler]

It's my pleasure to respond.
I hope your 2nd line of the thread was a typo and not a Freudian lapse to refer "useless" instead of the assumed intended "useful." It gave me a good laugh.
2 things:
1) Oxygen sensor
I'm guessing you will be pleased after changing it out. Make sure the new one works before installing per the test I did above. A dropped or mishandled sensor is a broken sensor...even if new.
The downstream oxygen sensor is also important...please don't ignore it.
Here is how the car works...for the 1st 2 minutes on old start up, the electric seconday air pump clicks on (pump is below alternator and is NOT belt driven), the air injector solenoid is activated under the black plastic shroud on the front your engine in back of the fan blade, the Secondary injector valve is then vacuum actuated to let the fresh air from the secondary electric air pump to enter into the exhaust.
Why?
The catalytic converter can only work if it has a source of oxygen to then make the HC and CO go to CO2 and H2O. The secondary electric pump supplies this for the 1st 2 minutes of car operation.
After 2 minutes, the solenoid is deenergized because the oxygen sensors (which work at 662F) have had a chance to warm up in that 2 minute period and actually work now. It's now up to the oxygen sensors to make sure there is enough extra oxygen in the exhaust stream to allow the catalytic converter to do its thing.
The pre cat sensor makes sure there is enough oxygen for the converter. The downstream cat sensor verifies that yes indeed, enough oxygen had been supplied because the downstream sensor wants to read zero oxygen. If the downstream sensor is not working right, the car's computer is going to be confused. The car will run like crap.
Replace both sensors. Please. If you don't, you can saturate the cat fine mesh to the point of destroying it. Worse, it can plug up. Remember the movie "Beverly Hills Cop" when Eddie Murphy stuck a banana in the exhaust of Taggert's police car and it wouldn't run. That will be you if the cat plugs.
One more note...my air injector valve that allows air into the exhaust for the 1st 2 minutes was recently rusted shut...vacuum would not open it. I had to remove it, soak it in Gumont and with a hand held vacuum pump prove that it would open after I rapped it with a hammer a couple of times. Black crap in the moisture coming out of my exhaust indicated I had a problem. A quick test with the hand pump proved the culprit.
Ask if that wasn't clear.
2) Transmission
I once owned a 1979 280E whose previous owner was fed up with it because the transmission was wierd like yours is now. I bought it for $400 and drained the transmission fluid and refilled it. It ran better. 1500 miles later, I drained it again and refilled and it ran perfect. Every 30K miles after I changed fluid and it continued to run great. Mercedes told the previous owner that they had to buy a new transmission.
Here's the deal...in the W202, the transmission holds about 7 quarts of fluid. Draining it gives between 4 & 5 quarts. Some of the old crap is still in there. 2 changes will get things circulated enough to get a pretty good fluid after the 2nd change.
Note: Crankcase oil has detergents...I can change crankcase oil and do other work on the car and have very dirty hands...I stick them in the old crankcase oil and wash up and my hands are incredibly clean albeit oily. Automatic transmission fluid is NOT detergent...yet by waiting so long to change its fluid it is gummed up inside the transmission (which is full of tight passages with springs and little ball bearings acting as check valves). Please don't tell anyone but on the 1979 280E inbetween the 1st & 2nd transmission fluid change I put in a solvent to clean away gummy deposits, then ran the car through all the gears for 15 minutes, then drained and refilled...yikes, did that ever clean up the gumming issue. Those long delays with harsh shifts is simply because the springs and ball bearings are so gummed up they won't move until really forced...then when they finally do, clunk, it's a harsh shift.
Fresh automatic fluid is mother's milk to a transmission.

[jnenad16]

sulaco, your hard downshift is most likely caused by a bad modulator
http://www.autohausaz.com/search/product.aspx?sid=zi1rpcvppldl2i552uakt0z0&makeid=800016@Mercedes&modelid=1195466@C280&year=1994&cid=33@Transmission/Transaxle%20System&gid=7776@Modulator

also, flushing a trans wouldnt hurt every once in a while :)

ps. I got your email a while back, but I was unable to respond due to personal affairs. so dont think I was ignoring you :)

[Sulaco]

Bill:

haha my apologies for the completely opposite word up there. I typed that up while at work yesterday, so I'm surprised that's the only error I made.

I ordered the pre-cat sensor only because of financial constrains. I'm still just a wee lad (27 and renting a townhouse). I don't even have a place to work on it right now. I can get under it to do the pre-cat sensor in my driveway, but the space/tools for the post-cat sensor aren't in my grasp right now.

nenad:

been a LONG time! glad to see you still post here, you also know quite alot about m104 w202's. I honestly don't even remember an email at this point, I think?

But after what both of you guys are saying, I guess I'll do a flush and swap out that modulator (I only recently learned there there WAS a modulator after researching the 240d's transmission). Could it be that the modulator's vac line is unplugged? or needs adjusted?

The transmission NEVER slips, it just has odd habits when it comes to actually shifting, and I've just ignored it for several (read: 5+) years.

Lastly: just follow the same steps taken on that 79 280E, Bill? Or do you guys know a particular way for a w202.

[BillKerler]

My 2 cents input is to do the transmission fluid change (twice, once now and another in about 1500 miles) as a first step.
Your thread tells the story to me...it's about temperature. When it's cold outside or when the car is not warmed up (cold start) is when your problem is at its worse condition. That's when gummy sludge is at its worse. As thingswarm up, the gummy residue softens up and allows the springs and ball bearing check valves to work inside the transmission.
If the fluid does not fix your problem, then investigate the modulator.
If you hear a vacuum leak, address it. Vacuum leaks are #1 priority to fix.
Disconnect a vacuum line and put a vacuum gage on it, you should have a steady 17 inches mercury vacuum at your Nashville elevation on a spring day weather.
If it's only say 15 or lower, you got a problem.
I use the MAF (what Mercedes calls the HFM for hot film mass) air flow sensor to tell me if I have an intake air leak anywhere. Pin #1 on the MAF plug in cap is + output voltage and pin #4 is to the car ground ( - signal). At 750 rpm, the reading should be .8 volt dc. If it's .7 volt for instance, you have a leak...in other words, air is getting into the combustion but it did not pass through the MAF (it got sucked in through a vacuum leak). This is the bullet proof way of knowing fo sure if you have a vacuum leak.

[Sulaco]

Alright. You've sold me on the transmission flush. The fluids i'm finding online (that sites direct me to use) are all like $12/ltr

What would you use?

[BillKerler]

I use Dexron III transmission fluid.

[jnenad16]

ATF available at costco will work just fine for your car, or any Dexron 3, wherever you can find it cheapest.

Now, to do a transmission flush, first, I would add a bottle of Seafoam Trans Tune and drive the car for like 5-10 miles, than drain the pan, followed by the torque converter, it will take about 15-20 minutes or more for both the pan and TC to drain completely. then drop the pan and replace the filter. the filter is held in place by 3 phillips screws. also make sure that the old cork gasket between the filter and the transmission comes off as well, otherwise you will have an improperly seated filter. put the pan back up. initially fill the trans with about 5 quarts of ATF. start the car and let it get to operating temp ~85C. DO NOT DRIVE THE CAR. after a few minutes, shift the trans through all the positions slowly, do this a few times, from park to drive and back. wait a few minutes. now add another quart or two, wait a few minutes again and check the fluid level with the car still running. you want it to be in the middle of the 80C marked range. add another quart if the fluid level is still in the lower 50C range or below. it usually take 7-8 quarts to fill the trans after a flush.

[BillKerler]

Sulaco - After you change out your pre cat oxygen sensor, be sure and disconnect your negative battery cable and clear out the volatile memory in your computer...anytime you do a significant change to your ignition, fuel injection, oxygen sensors, etc., clear out the computer brain by disconnecting power to it at the battery. Then you get to start fresh with the computer now that all your components are working right. Bill

[Sulaco]

Thanks Bill

I was thinking I'd do that. I can't even disconnect my positive battery terminal anymore. The cable connector is broken on it--so I will HAVE to disconnect the negative. I'm hunting a new cable, but can't find them anywhere (haven't tried dealership, I avoid them).

[Sulaco]

Forums are back!!

[BillKerler]

I borrowed a MacTool Leak Attack smoking tool...Medicinal grade mineral oil is burned in a device making smoke...using air compressor air, the smoke is pumped in throughout your car's intake and exhaust system checking all devices for leaks. I plugged the smoke tester into the air inlet after the mass flow sensor and I put a rubber glove over the exhaust pipe. I found a huge leak at the EGR valve closest to the firewall on the passenger side. The leak was between the EGR valve stem shaft and the packing gland around the shaft. Note, the EGR valve has a 3/8" tubing connection to it going straight to the intake manifold. Combustion air gets sucked in without having gone through the mass flow sensor...completely fooling the car's computer system for air/fuel control.
How much air leaked through? I used my computer launch pad that reads the onboard car computer and noted the mass flow sensor (MAF) reading with the leak was 3.32 grams per unit of time...after replacing the EGR valve, the MAF read 4.44 grams. Roughly 25% of total idle combustion air was leaking through the EGR valve leak. Yikes. No wonder my LTFT (long term fuel trim) was at -10.16%...my car computer with a 3.32 gram reading was showing such low combustion air amounts that it was telling the fuel injection to cut back fuel by 10%...that's terrible. I really had a lean mixture which is a natural way to produce HC and CO emissions plus rob power from the car.
Google search leak testing using smoke...this is a wonderful tool. I also checked my EVAP system with it.

[Sulaco]

That's brilliant. I wonder how much one of those tools costs?

Also, what do you use for your computer launch pad? What's the program? What does it use to connect to OBDII port?