300E Apparent Misfiring, Hesitation
 

1991 300E, 150K

This saga has been going on for nearly a year now. Intermittent "misfiring" at idle in D, stumble or hesitation immediately off idle for just a split second. Smooth everywhere else, including low speeds, coasting, and highway cruising. Happens mostly when cold, but also occurs when hot. I cured it temporarily a couple weeks ago when I did the famous quarter turn clockwise on the EHA screw (see thread below). This leads me to believe the issue is fuel related.

http://www.peachparts.com/shopforum/tech-help/104289-adjusting-eha-possible-post1242138.html#post1242138

New cap, rotor, wires, plugs
Cleaned ICV, hoses are tight
New valve cover gasket
Checked for obvious vacuum leaks = none found

The issue is most severe when starting cold, then gradually improves as the engine warms up. But it is never completely gone, and could resurface after an hour of driving, leading me to believe the coolant temp sensor is not at fault.

Since the EHA adjustment improved things temporarily, I'm inclined to suspect the EHA or (gasp) fuel distributor. How can I test these? Am I correct in understanding that the EHA is involved with controlling idle and low RPM mixture and that this control is handled by another component (fuel tower?) at other times? If so, this would lend to my theory.

When the "misfiring" and hesitation does occur, it really feels like an ignition problem, as if spark is not getting to some of the cylinders. But, having replaced the secondary ignition components, I can rule them out. Besides, wouldn't ignition deficiency be apparent at all times and not just at idle or off-idle?

I have searched and searched and nothing seems to make sense. As it stands, I suspect EHA adjustment, fuel filter or accumulator, maybe even coil. I have ruled out O2 sensor since this is occurring in open loop (cold).

I don't want to throw parts at the problem. Any ideas?

have you put a new fuel filter, do you use the same station for gas, do you use any fuel additives?

No new filter.
Same exact station for gas nearly twice weekly.
Use Techron periodically, last time about 3 weeks ago.

In my experience, fuel filter restriction is evident only when accelerating or cruising, as the demand for more volume cannot be met. My issues are at idle and off-idle.

I have gone thru some of the same issues and it is still not perfect but there is a difference in the way the car runs at some of the off brand gas stations even if you use the same one if mine starts any hesitation I go put cheveron or some times I will put a couple gallons of 100 octane, you go thru a lot of fuel is it all high speed or stop and go, mine does not like stop and go

Incredible - problem found. I just went out and started the car in the pitch dark, then popped the hood. The wire set looks like Tokyo at night - arcing everywhere. I was able to accentuate with a mist of water from a spray bottle. It was amazing, as in some instances there were 4 or 5 arcs forming on the same wire. Nearly every wire was arcing in multiple spots. The worst was the coil wire, which of course feeds the others. It's a wonder this car is running at all!

The worst part is that the wires are fairly new. I put them on just 2 years and 32,000 miles ago. They were purchased locally and were supposedly OEM - I believe they are Beru. I'm not taking any more chances. I want to buy the original wires that went on here, since the originals lasted 14 years and the car was running by the time I took them off. Any suggestions on which ones to buy?

Lessons learned:

1. Stick with the basics air/fuel/spark.
2. If you have a suspicion, follow that suspicion until it can be ruled out undeniably.
3. Test thoroughly (free) rather than throw parts at a problem.
4. Sometimes overanalyzing can be counterproductive.
5. Don't necessarily rule out new or newer parts as they can be bad.

performance products or fastline or ebay just make sure they are factory replacements

plug wires
 

i have found out the hard way that bosch plug wires don't hold up and that beru is the only way to go.there is a brand called kingsbourne that uses beru plug ends and they are good too. david poole dallas tx.

Misfiring?
 

Have you actually had the Lambda duty cycle (on/off ratio ) checked? If not you're shooting in the dark!
Also check or have checked the maximum coil output, If lower than 28KV that may be the problem.
If you don't have the tools take the car to a reputable shop and ask them to check coil output and on/off ratio so you have a place to start!
Good luck! Mike

I see you are new here. Did you even read the whole thread? My problem is the ignition wires which are arcing like a Christmas tree.

I had Beru on there. Those are the ones that did not last. I believe the original wires that I removed were Bosch. They looked to have been on the car since day one, as they were all old and corroded and were stuck to the sparkplugs (I had to break a few plugs to get them off). But that was 2 years ago and I have slept since then.

I just located a set of Bosch for $102 locally (sorry Phil) and will be picking them up at lunch.

wires
 

that's very strange because oem is....beru....aftermarket is bosch david poole.

Now you have me worried... Well, either way, what can I say. Beru burned me so I'm trying Bosch. These have a lifetime warranty from Autozone, and believe me I'm saving the receipt. Even if I did have to pay $102 to buy another set in 2 years to keep this car running, that's OK with me as most people pay 3 or 4 times that amount every month on a car payment (not to mention the full coverage insurance). In 2 years and 31,000 miles I have spent less than $1,000 in maintenance and repairs (including tires).

OEM was Bosch at the time. Bosch and Beru bought today do not spec properly in the range of 800-1300 Ohms end to end. Get NGK they are 980Ohms right out of the box and hot they only wander to around 1050, well within the range and almost perfect considering spec is 1KOhm.... I went through several sets of Bosch and Beru wires and ends and none of them ever tested 1KOhm cold or out of the box. The lowest I got was a set that was 1.1KOhm cold and 1.28KOhm hot, too close to that edge for me. Those were Beru. Bosch all were over 1.3KOhm when at operating temp.

GMer-

Thanks!

I have a 1991 300E that has been giving me fits the last few weeks. I bought it cheap because of starting problems. I took it to my mechanic who said he had it all fixed.

I picked it up and just knew something wasn't 100% on it. After about a week of driving it, it stalled and would barely restart. A tow bill of $100 and a day later he says he has found the problem. A bad ignition coil. We replaced the ignition coil; however, I still didn't feel it was 100%. In searching the forums to try to find anyone with similar problems, I came across your post. After reading your post I thought back to a car my Dad had years ago that was firing thru the wires. So, in the pitch black of night, I raise the hood. Sure enough!! It is firing thru the wires looking like a blue light flashing under the hood! I'll replace the wires, and maybe the cap and rotor this week.

It really irks me that my mechanic never thought to check these wires in such a simple way. It would have probably saved me a tow bill.

I think you are definitely right. Stick to the basics. I think we get caught up in searching for some exotic probel that we overlook the obvious.

Again, thanks for posting this thread!!

Fred McIntire