Hey all. Quick question:

Yesterday, I changed the water pump on my 1997 C280 (88,000 miles). Straightforward job, no real drama.

Today I did a few local test drives (total of 50 miles or so). The car started, re-started, and ran well. Then, all of a sudden, the idle went rough and the CEL (check engine light) came on. I *assume* it's related to my recent work.

Notes:
- I checked the tiny vacuum hoses and electrical connectors in the area of the water pump. All seemed intact.
- I did not bleed the system of air because I could not find a bleeder valve, so I *assumed* that is not necessary. Can an air pocket trigger the CEL?

Any relationship between a new water pump and the CEL?

Your advice is much appreciated.

Air pocket will not trigger the CEL, just blast your defrost for a while and then recheck the coolant level, that's a quick way help eliminate air pockets.

Did you install a new thermostat? I imagine a stuck open/closed thermostat would set the light. But the most common fault with that engine is the original wire harness. If you disrupted the old crusty harness, you may have broken or cracked some of the wires. That is a common cause of check engine lights that immediatly follow engine repairs -at least on the 104 engine.

Thanks for the input. I will look into it. Yes, there were a few bad connectors. I cleaned them and put shrink tubing over them for protection. I will reinspect them.

By the way, any off-the-shelf anitfreeze recommendations that will work with these cars? At 90K miles, I was ASTOUNDED how clean the inside of the old water pump was. NOT ONE bit of old accumulation -- just shiny aluminum. Wow.

Any other ideas, I am all ears.

Thanks!

It would help if you could retrieve the stored trouble code (s ).

__________________
2007 C 230 Sport.

Capturing codes is beyond my ability, I am afraid. My tool box is pre-OBDII.

I reset the computer and will take it for another drive to see what happens. It previously started and ran beautifully. Turned it off. Seems something happened during the heat soak. Restart was a little rough. It got worse. CEL light then came on.

Isn't there a way to get codes through the AC control? I will have to search for that.

Capturing codes is beyond my ability. My tool box is pre-OBDII.

I reset the computer and will take it for another drive to see what happens. It previously started and ran beautifully. Turned it off. Seems something happened during the heat soak. Restart was a little rough. It got worse. CEL light then came on.

Isn't there a way to get codes through the AC control? I will have to search for that.

Time to upgrade your toolbox.
http://www.iequus.com/item.asp?cid=10&pid=3100
It works on all 1996 + vehicles, not just M-B's.

__________________
2007 C 230 Sport.

You're right. Time to add some silicone chips to my tool box. Thanks for the advice. -Bob

check the area around the pump, see if you may have knocked a vacuum line loose, or maybe one is craked, the vacuum fitting for the air pump and egr vale is right behind the pump on the intake manifold, if they werent working, that would set a code.

Thanks. Yup, I did that ... twice in fact. Everything looked good. Then I reset the system with a battery disconnect. As I was doing that, I thought about the gas cap seal (common overlooked OBD2 CEL problem). Hmm, it seemed a little "iffy." I reversed the rubber seal, put the battery terminal back on.

After a day of driving around town, with lots of stops and starts, I saw no CEL. Thought I was out of the woods. Then it happened last night: all of a sudden, the revs dropped, it started running rough and the light came on. Damn!

I'll have to assume that the new water pump and the CEL coming on are just coincidental things. Still seems odd though. Maybe I'll go back and check those wires (and those tiny vacuum lines) one more time.

Besides this product, are there any recommendations on the very latest in LAPTOP-based OBD2 software and plug-in cables? Given the ability to update the software, I figured it'd be smarter/cheaper to go the laptop route than to buy a dedicated hand-held device that cannot be updated. (Maybe I am wrong about that.) I'd also like to find something that is not marque specific and has a useful lookup table to interpret the cryptic error messages.

Thanks.

bob, i just noticed you are in sandiego, send me an email, i too am in SD, i will look at it for you, we have the latest mercedes diag software. Ray

Update. I took Ray up on his kind offer. I stopped by. Nice place! If anyone wants work done in, or even near, the San Diego area, you should contact him at mercedessd@cox.net. He knows his stuff (as if you didn't know that by now).

Ray read the error codes for me. The report is that cylinder #2 is misfiring -- or not firing at all (affirming my feeling it was running on five.)

I drove back home, stopping at Inter Auto Parts, a shop supplier, to get new plugs and a coil (#2, #4 and #6 have coils). Inter Auto had the plugs but did not have a coil (they're backordered from Germany). Note: Inter did not have the exact plugs from my owner's book (Bosch F 9 DCO), but told me there is an updated cross referenced to a Bosch F 8 DC4. Hmm. The plugs looked the same, except the new ones have a crush ring, whereas the old ones did not. I would have rather put the OE plugs in it, but I assume Inter knows their stuff.

The car ran poorly all the way home. Had to keep the revs up because it was running on five. I hope that did not stress the catalytic converter.

Got home, opened the hood, got to work. The old plugs have about ~20K mi. on them. They all looked in good condition (light tan), although the gaps were a little big. The suspect plug, #2, seemed a little odd though. Its aluminum tip, at the top where it connects to the plug wire, was oxidized -- you know, dull and gray with a slight powder on it. Maybe that is an indication of a spark arching? The aluminum tips on the other five plugs looked normal.

I gapped and installed the new plugs. Next, I pulled the wires off the coils, checked everything, and put everything back together. Although I was thinking of it, I did NOT move the coils around -- all three went back in the same location.

Note: when I removed the air plenum that goes from the air cleaner to the throttle body, I looked down in the area of the exhaust headers and noticed that the large rubber elbow (approx 3/4" dia tube) that connects the secondary air pump had pulled away. The system was now was open to the atmosphere. I don't think I disturbed it when I pulled the plenum off -- the elbow tube was pushed up, not down. If it had been me, it would have been pushed downward. I assume an open line like that could make a CE light come one, but would not cause #2 to misfire. Anyway, I refitted the tube and moved on.

Last thing I did was disconnect the battery, give it 15 minutes, and reconnect. That got rid of the CEL.

The car started fine and runs very smooth. We'll see how it does after getting hot.

Again, thanks to this list and Ray. You guys are great.