My 1985 380 SE (w126) with just under 100,000 miles on it was working well yesterday, until at about 70 mph I nudged the accelerator and the car started to vibrate as if the motor was missing or pinging.

I pulled off the hwy and at idle it was missing too, but less. Once I pushed the pedal down to drive off it stumbled quite badly.

My thoughts are:

1. I run the car on 91 octane premium fuel. By accident, I addad about a 1/2 tank of 87 octane that morning to the 1/4 tank of premium already in there, about 100 miles before it acted up. As soon as I pulled of the hwy, I topped up with a 1/2 tank of 94 octane Sunoco. The problem did not go away for the remaining trip home. What does a type 116 3.8 liter North American V8 engine need for octane? The manual says 87, but....?

2. I bought the car recently but have no receipt for any service where the previous owner changed the fuel filter. The pump motor seemed sort of loud a day or two before. I will change it now, bit has anyone ever had a fuel filter plug up? What happens? I will pull the plugs and see if they read lean, but..?

3. It was a rainy night. I guess I will check the distributor, rotor, wires, plugs, etc., but if it were getting wet, would the whole thing not just pack it in? I know my car has a screwy idle control computer, but I doubt that would cause it. I guess I will check it again when it is dry.

Any ideas?

I have had the fuel filters on both my 1979 Alfa Romeo and 1993 Daewoo plug up before and the symptoms were the same.

On both occasions, the mechanics (different workshops) did a ridiculously similar thing, that is, blowing into the fuel filter (in the opposite direction of the fuel flow) to clear up the plug and that solved the problem on both occasions.

For what it's worth, my $0.02....

Good luck!

It is not usually load that decreased fuel flow effects, it is RPM. As the RPM increases, a plugged fuel filter prevents adequate fuel flow, so it intermittently stumbles.

That said, you should go ahead and replace the fuel filter since you don't know when it was replaced last. It very well could be your problem.

Good luck,

Another thought, could it be a collapsed / blocked catalytic converter? I noticed the car was a bit louder when I got out of it, when it would stumble there would be a popping eminating from the middle of the underside of the car. Are these pricey?

did you say catalytic converter? you better hope not.
i am sure the cost (OEM) will be more than what you paid
for the car! good luck.

I tried to check the cat converter last night. I have been told that if they break internally, they will sound like tin with gravel in it if you tap them, and they can even glow red where the blockage is. I have found no such symptoms. The exhaust seems to flow well enough too. The car still does miss, but only under load, and only when warm. I still think its fuel delivery (filter or something wrong with the pump) but that is just a guess.

I checked two plugs as I had little time before it got dark ( plugs were malt brown/tan and healthy looking) and I could find no flaw or significant wear on the distributor. If a vacuum leak were to occur with the vacuum advance on the distributor would that cause such an issue? It almost seems as if the ignition cuts to one cylinder when under load, any clue what would cause that to suddenly start happening?

I love this shotgun approach to problem solving.

At any rate, the best price I have seen for a new non-oem cat is US$599, but I can get a used, good condition one for CDN$400. Here in Canada there is no law against reusing such parts.

had the same problem with my SEC.Problem was bad ingnitin wires . Was not a cheap repair bout $150 for oem wires

I agree with 500sec, that definitely sounds like an ignition problem, not fuel. Have you washed the engine lately? I'm thinking water contamination of your ignition system......

Dale

You quit too soon when it got dark. Engine running in a dark area is one way to check for failing insulation on your plug wires. Darkness wil allow you to see arcing to ground along the wires. Sometimes the wires just breakdown internally. Usually you can test for this with an VOM. Bad plug wires are a classic cause for the symptoms you described. Get the real wires, do not fool around with aftermarket wires. They just don't last. Maybe you should think about a new cap and rotor. Might as well do the whole job right.

The diagnosis is complicated by a few factors:

1. The day it happened I was using 87 octane, not 91 for the first time (though I understand that the car should run OK with 87, and I have now put gas line antifreeze and water remover but it didn't help).

2. It was raining pretty hard and I don't know how well the sealing rubbers work on the leading edge of the hood (but it has been dry since then and the problem still exists - I did get the car rustproofed and they did spray in cavities around the motor, but that was 2 weeks ago?)

3. I have pulled the cap and rotor they seem OK, and the plugs are new and seem to be a healthy tan shade. I did check it out in the dark, and observed no sparks.

4. It only screws up under load, in neurtral I can rev it out, but it only takes a light touch on the pedal to do it.

5. I did unplug my idle speed controller box (mine needs a repair) and aside from theidle jumping to 1500 rpm, the car ran just as poorly.

I am going to do the filter tonight, any other clues? I may get someone to sit in the car in drive, get it to misbehave and check the spark with a timing gun.

I don't know what else?

Antone know where the idle speed controller box is on an SL? I may wish to swap the out just to see.

thanks for the suggestions,

John

Misfiring during acceleration does sound like an ignition problem. Fuel problems usually have a flat, no power feel. I would definitely check the ignition wire resistance. I had a misfiring problem with my Camry and replaced the fuel filter, checked every fuel injection and emissions component and when I finally tested the plug wires they had infinite resistance. I can't believe the car ran at all.

I'm not sure if your car has resistor wires or resistors in the spark plug boots like my 260e but either way I would check them out.

If this engine has a points and condensor ignition system the condensor may be bad.

If it does not have a points and condensor ignition system then never mind!!

I have put a timing gun on the car last night, the ignition advance seemed to work OK in neutral, as revs would rise, the igntition timing would advance, so at least there is no vacuum leak to the distributor advance.

I may get a fuel pressure test done on Saturday, just to rule fuel delivery out. The fuel filter is somewhat stubborn, the nuts on the bracket have rusted and are coated in liquid wrench waiting for me to get home.

I will also check the resistance on the ignition wires and spark plug caps when I get home tonight.

Anyone ever have any problems with their warm up compensator? I would have thought that it would provide a consistent problem, not just one that would show up under load. Another potential souce of problems could be the pressure step switch located in front of the windshield wiper reservior beside the horn, which was added to 1985 cars. It richens the mixture under acceleration on 1985 US spec cars. Anyone ever seen one of them fail?

I also note that the problem only surfaces when the car is warm, after it has idled for a while.

I took myself to "school" last night on all 44 pages of the CIS-E fuel injection system as set out on the CD Rom. Complex, isn't it?

A good way to check for obstructions is to connect a vacuum gauge to the intake manifold, accel to about 2 grandwatch the gauge as the vehicle decells, it should increase vacuum immediately when the throttle closes then stabilize. If there is blockage, the gauge will will show no increase

Thanks, I have a gauge for synching the carbs on motorcycles, I will try that tonight as well.

Is there a test fitting on the manifold somewhere? I guess I could just unplug a vacuum line somewhere, but where would be the best place?