300E 1988 misfires

[ACM]

I know that it can be 102 different things, but here is the diagnosis.
I changed spark plugs and wires about a month ago when it first start doing it. There were cracks on the wires. I put Bosch plugs and wires in there so the quality should be okay.
The problem dissapeared.
Now, almost a month after the replacement of the wires and plugs, it start doing it again.
On any stoplight i have to put it on neutral because the shaking is being felt a lot.
Any ideas?
thanks for any input

[haasman]

If it were my car I would check the cap/rotor and the coil and its connections.

Haasman

[ACM]

can it also be the OVP?
how do you test this thing with a multimeter?

[ACM]

okay I read in the MB manual that in order to test the diode of the OVP, you connect a multimeter across terminal 31 (+) and terminal 30(-) and observe the voltage reading. I got 0.522 volts (bouncing a bit), which theoretically means that the diode is okay. However I want to open it inside and to check the solder connections. The number is 201-540-32-45 with one fuse and 2 metal contacts at the top. What are the contacts for????????????and how to open the casing??? it seems that these contacts are holding down the black cover.
any ideas

What do you mean by misfire? If you mean that it seems that one cylinder is giving up then the next step is to do a compression check or better yet a leak down test.

Good luck,

[ACM]

well, I checked the spark plugs that I replaced about a month ago but there were carbon depostis on all of them (some more some less). So after some cleaning of the plugs the engine is roaring again.
This probably points out that the fuel/air mixtyre is rich (right??) which can be caused from a bad oxygen sensor (right??).
Replacing the O-sensor should be a piece of cake as I recall from readings.
Is it anything else though?
How do I test the fuel mixture of the car?
and also how do I read the codes from this car (an 1988 300E)??
thanks for your input

Your thinking is correct, it is indeed running rich unless the carbon you saw was oilly rather than fluffy. If it was oily then it is pumping oil. A very weak ignition could be causing something similar.

Unfortunately I am not versed in your particular injection system, but the O2 sensor is indeed one possibility.

If this is a KE injection system, then you need to disconnect the EHA and see if the engine runs okay. If it's not a KE then I can't offer any knowledgable help. Sorry for my ignorance of the breed.

Good luck,

[dave_rose69]

ACM: Firstly, I hope you installed the correct spark plugs in your car, only use what your owners manual specifies, none other. .032 should be the gap. Never use a platinum multi electrode plug, they are mostly all resistor design and you probably have resistor equipped plug caps on your wire set. (metal caps) You dont need two resistors on each plug! Take the last HT plug lead nearest the firewall and put a new test plug on it, ground the plug to the cylinder head and do this in a darkened area. Your spark should be whiteish blue and not redish orange. The little coil body that MB uses in these cars rarely last more than 10 years especially in rainy areas. You will find it better to order a new coil from www.*************** for 66.00 bucks, and you should also change the OVP relay. (this coil comes in a left and right side mount, so check yours visually before ordering) Only the best of electronic technicians can refresh that relay, and if you dont clean it up right, you'll add more problems for yourself. If you can get it out of its aluminum case without breaking it: 1. Clean and Reflow all solder joints, 2. Burnish both contact point surface and blow out grit, (very hard to get to) 3. Deoxidize base connector pins, 4. Retension fuse holder contacts, 5. Test resisitance through the newly serviced contacts with a digital VOM, 6. Reseal and water proof encasement, only then are your good. If not, $60.00 for a new OVP sounds much better. Let us know how you make out.....
Dead Giveaway to your problem is: Car ran better after wire change, then started running bad soon after. Ignition coils get gradually weaker and weaker.....
Dave...

"93" 300TE 4 matic 187,000
"85" Volvo 740 Turbo 166,000
"85" Honda Limited Edition Gold Wing 1200 17,000 miles

[ACM]

thanks for the replies Dave and Larry
I will check and/or replace the coil ASAP.
Looks like after almost 20 years, it is time to do so.

BTW, anybody knows how to test for lean/rich mixture
and how to test for the codes for this car?

You are doing the check for a lean or rich mixture by reading the spark plugs.

Another way would be to monitor the O2 sensor output while driving with a DVM. I would have to look it up, but I think that it is 451 Millivolts that indicates correct stoichiometric ratio. More voltage than that indicates a rich mixture and less indicates a lean mixture. The voltage vs. fuel/air ratio relationship is EXTREMELY none linear.

Good luck,