1988 300TE Misfires -- Let's get this fixed!
 

I am going to summarize the original thread as its very long and hopefully I will be able to get responses now with everything summed up.

The misfire is random and inconsistent. We have pulled each spark plug out separately and the car misfires harder, but the same amount with each removed, so concluding the injectors/spark plugs/distributor are ok correct?

Firstly, compression rates:
Parts replaced/steps done so far

• OVP
• Coolant Temperature Sensor (checked and ECU adjusted accordingly)
• Voltage Regulator
• Injector's+Seals (previous owner)
• New Fuses
• Non-Resistor OEM Bosch H9DCO's
• New Bosch Ignition Wires (Misfires were now not as strong, but still present)
• Distributor+Cap
• Fuel Filter
• Valve Stem Seals
• Breather Hoses
• Fuel Distributor
• Ignition Coil
• Fuel Pressure Regulator
• O2 sensor
• EHA Valve
• --Checked for Vacuum Leaks (also by 3 Indy's)
  -----
• I have taken the car to three separate Mercedes Indy's in California and none of them could tell me what was wrong so far.

Exhaust smells normal. I sprayed WD-40 around the injector's to test for leaks, but there was no change in engine speed.

We have set the duty cycle to 50/50, 30/70, 70/30 and it didn't make any difference so we have it 50/50 right now.

What's next to test for?

Here's the condition of the engine:
http://i135.photobucket.com/albums/q...alveSea2lE.jpg

Have you checked to see if the fuel distributor is delivering a properly balanced fuel supply to all the cylinders?

They do go bad and cause the symptoms you describe.

Very nice and clean engine.

Following up on a real good possibility suggested by dpetryx,

You can check this:

Disconnect output lines to all the fuel injectors from the fuel distributor.

Bridge socket terminals corresponding to terminals 30 and 87 of the fuel pump relay. Engine is off, but ignition swicth is on to get the fuel pump going.

When the air sensor plate is on the top, no fuel should flow from any of the fuel distributor outputs. Then when you gradually press the plate, equal amount of fuel should flow from all the outputs. If this does not happen, need a fuel distributor replacement.

I replaced all the injectors in ours for some symptom I no longer remember. However, there was no concrete improvement, so I did not discard the old ones. About a year later, a misfire developed - probably more consistent than yours - and I put one of the old injectors in the problem cylinder. That was the problem. Moral is that the injectors can randomly fail, so unless the PO was trying to fix this same incident you now are dealing with, you had might as well buy a new one - pretty cheap, here - to use as a diagnostic for that problem.

Steve

Yes they were trying to fix the misfires too. They couldn't figure it out and with a newborn child, couldn't risk having a car which misfires and stalled(we fixed the stalling due to bad OVP).

Will run those tests on the Fuel Distributor this weekend.

Thanks everybody!

bumping the thread. I'll keep everything in here now.

Just replaced the fuel distributor a few weeks ago and it made no difference.

I'm getting really stumped now as I have replaced a lot of parts which have so far not fixed it.

Do you guys think this is electrical or fuel? I am going to go back and double check resistance in the new plugs due to the fact that replacing them did help, but did not fix entirely.

I vote for spark - take another look at the cap and rotor. Is there a cam sensor on these? What triggers the spark? Yes, I definitely vote spark.

Good luck!

I just replaced them not even a week ago, both brand new.

I'm not sure about the cam sensor.

Crankshaft sensors can cause a problem like this. I believe it is at the left rear of the engine. A scope is necessary to check this to be sure. I have found the signal is weak at idle but good at higher RPM causing a random miss at idle but run OK at higher speed.
Just a thought.

I spoke to a different mechanic on the phone who I was recommended to for his knowledge of the 124 (Bob located in Long Beach, CA)....He sounded very knowledgeable and said its most likely fuel related as its just at idle.

If my fuel pressure regulator does not fix the problem, I'll take it to Bob first thing friday morning and at the very least get him to do a leakdown test for me. Maybe I will have some luck on the 3rd indy this time round huh? 3rd times the charm!

Generally if the fuel pressure regulator was going faulty the diaphragm would leak and you would notice petrol exiting up through the small hose to your air filter. You would notice a fuel smell.

Have you changed the fuel filters and accumulator? Have you had the fuel system pressure tested? Fuel pump relay? (intermittent fault)

How are your lifters?None noisy?

Just adjacent your IACV on the manifold there is an grounding junction point. If you have corrosion on this point or the bolt holding the wiring down is corroded this can have an impact on the electrical parts of the injection system.

If you disconnect the wires from the EHA and IACV this will take the electrical portion of the injection out of the equation. The car will run on pure fuel injection with no electrical adjustments and the sensors become ineffectual. If you do this does the engine still misfire? At idle the engine should rev up to around 1100 rpm. - No iacv controlling the idle.

The engine must have no misfire otherwise the sensors will give faulty information to the ECU. This will inturn make the ECU send incorrect information to the ignition and fuel controllers. Your plugs, leads dissy, rotor and engine condition must be good.

Your engine is very clean. Generally if the valve stem seals have gone then there should be some crud and dirtiness around the valve and rocker lifter area.
http://farm3.static.flickr.com/2210/...a368f7dbc2.jpg

Fuel filters have been replaced, but not accumulators. Fuel pressure has not been tested (I will ask Bob to do that tomorrow for me when I take the car in)

The stem seals were bad because the spark plugs were covered in oil, but we fixed that and the plugs are perfect now.

I unplugged the IACV before, but the car just died if I didn't keep the RPM's up.

I just swapped out the Fuel pressure regulator and it feels a little smoother although the misfire is totally inconsistant, so I don't know for sure until I drive it for a few days.

Hmmm. Should be harder to start but should idle around 1100 rpm with the iacv electrical lead off. You have replaced everything else -- AFM??? From your picture the AFM and damper look pretty clean and new.

When you replaced the Fuel distributor, did the rubber seal for the AFM go back on the throttle body properly? A leak here will cause problems.

Ok. Just got back from taking the Car to Bob located in Long Beach, CA.

He was very knowledgeable and ran numerous tests but couldn't find much initially. Then he unplugged the EHA valve and found it made almost no difference to the idling condition which was strange -- meaning the EHA is either A) Not functioning properly B) Not receiving correct information from the O2 sensor.

And B) is the most likely cause as I have said before that the O2 sensor must be tarnished from the oil consumption that I had before (even though it is working because I am getting volts out of it). His machine although said that the O2 sensor's readings were a little out of whack.

So replacing the O2 sensor is my number one priority now. I think that we may have solution in hand!!

What's the best thing I should do now? Buy new/used or get a 1990 mustang 3 wire bosch sensor (I understand they work) from the local store?

I have another EHA valve that came with my fuel distributor, but Bob said its highly unlikely that the EHA has a problem.

Also, are there any articles online on how to change the O2 sensor? My Haynes manual does not mention it anywhere. Is it difficult?

If you do a search on this forum you will find
(a) how to test the O2 sensor
(b) how to change it.

If you cant find this info on this forum - google it.