Where to go next with Misfiring 300TE
 

Before posting give a quick read of whats in this post please it will take only a minute at most!! I get too many of the same responses.

Car is 1988 300TE with 87,000 miles right now.

Compression within 1bar at 185 +- 5

Parts replaced:
* OVP
* Coolant Temperature Sensor
* Injector's+Seals
* Non-Resistor OEM Bosch H9DCO's
* Ignition Wires
* Distributor+Cap
* Fuel Filter
* Valve Stem Seals
* Fuel Distributor
* Ignition Coil
* Fuel Pressure Regulator
* O2 sensor
* EHA Valve
* Smoke tested for vacuum leaks


Where should I go from here?

I have no problems accelerating and it only exists at idle. I had my local mechanic smoke test it today so vacuum leaks are off the list!! There is no check engine light on, or any error codes.

Rpm's bounce about 50 at idle....It can't be mounts as sometimes it idles perfectly fine, but other times its horrible....very variable...and along with the rpm's bouncing....its not related. You can see the engine is shaking + surging when you pull the hood and doesn't sound as happy as it could be.

He mentioned that I have a leaky thermostat housing, but that shouldn't be causing a misfire as my coolant temperature is fine....but I've ordered the gasket to fix that soon anyway.

It may be normal for your engine, the early 103 engines have camshafts with more duration that the later cams and do not idle as smooth. If you have ruled out everything else you may have to live with it unless you want to spend a ton of cash to replace the cam and followers.

Its an actual misfire, I'm pretty sure its not normal. I have spoken to other people (iwrock for instance) who also has an 88 TE and his idles smooth as butter.

If you are certain that it is a misfire then I would tend to think it is ignition related. Have you actually had the ignition checked with a scope to visually verify the coil output and oscillation? The only things that seems to be missing from that impressive list is the crank position sensor, ignition module, and the harness from the ign module to the coil. I've only seen that harness bad once but it was a real ball buster to diagnose. If you can put your hands on a known good ign. module it would be worth it to try. The sensor is a long shot but you've seemed to have eliminated all of the "short shots"

You replaced all the parts I thought might have been bad... :confused:

I'll look at ebay and see what I can dig up. I'm not worried buying used because if it makes a difference, I'll know its that unit. The chances that I buy the ICU with exactly the same problem is slim to none.

I will check coil output (coil is new though) and oscillation.

I was going to do the CPS but I decided to go ahead and get her smoked before I spent any more money.

Have you tried taking it to Mr. MB Motors in Tarzana?

Ya I took it and he didn't fix it. I was actually kinda surprised. He told me it was a vacuum leak (which it wasn't) and He replaced the suspect hose and it ran a little better but I lost all my power. Ended up that the EHA valve was causing the loss of power...but once adjusted my misfire came back....so it wasn't a vacuum leak at all. He said it would never run better than this as this is as good as that car ran new....

...Don't believe him at all. The idle is horrible so I was kinda surprised based on his reputation.

Could it be an exhaust issue. Like a back pressure problem?

Just guessing here :)

Or maybe even a worn wiper arm under the fuel/air meter under the throttle body.

You ought to take it back.

I don't know if your model fell into the engine harness problems, I think the '88 was before that, but you might check it out.
Ed

Ya no wiring harness problems for my car....the harness visually looks fine....unless there is an easy way to establish that it is okay, I will probably leave that til one of the last things to do as its so unlikely that its bad considering my miles.

He is already 70miles each way....I found another mechanic close by who seems good so far.

Well if I had any back pressure, I would have problems, or have hints of problems at higher rpm's....which I don't have at all...if anything it seems to pull harder when getting into the upper rpm's.

As I said, I am not sure if your model year was in the years with the faulty harness. When they did fail, it was the interior insulation, the exterior looked excellent.
Ed

The 103 engines do not typically have harness problems. The harness is typically only an issue on the EFI cars (104, 119), not the CIS cars. Then again, anything is possible.:rolleyes:

The only culprits left that I havn't touched yet are the EZL, ECU, Crank position Sensor and fuel pressure.

I plan to do the fuel pressure first once I can nab the bosch adapter to test it via the distributor. I can find cheap used EZL's on ebay for about $10-$30 so its worth a shot. I'd buy the crank pos sensor new though for sure.

Just waiting to move house so I can get back on track...

Is this a random misfire at idle only. I have found on my 500SEL that if I get a random misfire it will be plugs or plug wires, or lean condition. The lean condition can be temporally corrected by adjusting the mixture to a rich condition and just see if the misfire goes away. Problems I have usually found with the plugs are the wrong plug for the engine. Don't depend on what the dealer or any other store sells you, but find the original type plug for your car. Also, check each plug wire with an OHM meter and make sure they are all reasonably close to each other. On my 500 they will be aprox 5K OHM. If the above doesn't show up anything, then replace the CPS. The signal from the CPS is lowest at idle, and increases with RPM. If it is getting a little weak, it will cause your problem. If I was working on this car, I would research the plugs first, then replace them even if they are the correct ones in the car. I work on cars with this type problem every day and most misfires turn out to be plugs or wires.

Well with quite a bit of experience of my own 190s, the only time I've encountered this condition, and it's very repeatable, is when there's an air leak somewhere. On my 2.0 for a while the rubber pipe came loose off the valve cover (it goes to the air intake). This made the whole idle incredibly shakey but once over 1200rpm it ran as brand new.

I can do the same with my 2.5 - unplug the air pipes for teh air enveloped injectors and it's instantly huppity wobble jigga jigga.

Yes I have the OEM H9DCO plugs in her. I have been through tons of plugs too before I got my valve stem seals fixed. So I'll check the ignition wires then replace the crank position sensor.


I had the car smoke tested...so I have no leaks.

OK, not sure what that is but you say it's a test for vacuum leaks but I'm talking about actual intake leaks - extra air getting in and it being unmetered by the air flow flap?

While we're there - does the air flow flap sit in its rest position correctly with engine off? Should slide up into its hole, not catching on the edges at all.

Yeh its fine.

http://i135.photobucket.com/albums/q...alveSea2lE.jpg

^ You can see how good the engine is. Its gotta something small. Once I have checked out the wires and fuel pressure that CPS is going first!

ooh ooh ooh.... Throttle position switch? Either the one on the linkage with the wheel, or the one one in the intake but with the pins at the side which you can test for full throttle or idle throttle positions. Got to be said that you should test connection through to the ECU as well, have heard of problems with wires between ECU and sensors.

incidentally engine will run without ECU.

You're right. I believe all I need to do is pop off the OVP and that turns off the ECU. Last time I tried that was when I was checking for vacuum leaks using the O2 sensor...it still misfired but I've progressed alot from then so I should give it a shot again now.

Take a look at these wire readings:

1) 1.266 k Ohms
2) 1.271 k Ohms
3) 1.248 k Ohms
4) 1.263 k Ohms
5) 1.227 k Ohms
6) 1.107 k Ohms

They look pretty linear except Plugs 3 and 4....Are they within spec or could they be suspect? These plugs are about 6 months old since I bought them new. OEM Bosch ones.

What do ya think?