Engine missing 450 SEL - PeachParts Mercedes-Benz Forum

milterino · #1 11-09-2005, 12:12 AM

When I accelerate at say 20 mph the car stutters untill I getup to say 30mph. To me it is like a valve is burned but the compression for the cylinders comes out with them all about 10% of each other. If a valve was burned it would affect the compression wouldn't it? The no. 1 cylinder plug soots up on me and the no 8 and and 4 have a tendency to run sooty or oily. I put a tablespoon of oil in the cylinders and the compression jumps up by about 10 or 15 %. All the plugs are firing as indicated by putting the induction sensor on the wires. Is there such a thing as the wire firing at idle and not firing under load? Any suggestions would be appreciated. Milt.

BradleyP · #2 11-09-2005, 09:14 AM

My 72 280SE had the same sooty/oily issue on #1, and when we pulled the plug for the first time, it was totally coated. It looked like a mud dauber built a nest on that plug.

Needless to say, it was time for a head gasket, and since they were going to have the heads off, a valve job. The valve guides on three of the cylinders weren't up to snuff, so I had them put oversized guides in all the valves.

Be prepared to either invest a lot of time and/or money. A good shop here in CA will charge you $3500 and take a week to do it, a bad shop will quote you $2000, charge you $3200, and take six weeks.

BradleyP · #3 11-09-2005, 09:16 AM

P.S. All of my cylinders were reading within 5psi of each other before.

GaryF · #4 11-09-2005, 10:09 AM

Is your ignition timing advancing and retarding correctly ?

230/8 · #5 11-09-2005, 10:18 AM

If the rear cylinder spark plugs are sooting up you might also look at the transmission modulator. It may have a leaking diaphragm that allows a bit of trans oil to migrate to those cylinders. The vacuum line for the modulator is at the rear of the engine and those cylinders would be first to see any oil coming through the line.

230/8

MercyBenz63 · #6 11-09-2005, 11:13 AM

Your question about the wires is a good one. It IS possible for the wires to carry current at idle and degrade under load. A miss under load could easily signal the death of the wires, and for most Mercedes with orginal Beru wires this happens between 100-150k.

milterino · #7 11-10-2005, 12:02 AM

Your answers pose more questions. Bradley, if your cylinders are all within 5 psi of each other why would a gasket be indicated? Gary, the advance works OK, the retard doesn't work because the retard diaphram has deteriorated I assume. 230/8, how would I check the modulator, take it off and check it with my newly purchased "Mityvac" to see if it holds vacum? Kyle, do you know what the process would be for the wires to deliver at idle and fail under load? Seems like the coil delivers a specified voltage whether at idle or at higher rpm. Am I wrong? Does anybody know of a good cheap way of testing the wires before I go out and buy an expensive new set? Thank you for taking the time to answer me. Milt.

ctaylor738 · #8 11-10-2005, 08:23 AM

Missing under load is a classic sign of something amiss in the spark world. Cap, rotor, and most likely wires. Another possibility is leaky injectors fouling one or two plugs which then fail under load.

So make sure the cap and rotor are not corroded and put on a new set of wires. If you want to make sure of the wires, see if you can find one known good one to replace #1 and see if that make a difference.

230/8 · #9 11-10-2005, 01:10 PM

A few ways to check the modulator: visual, is there any visible oil in the vacuum line? If so, the modulator is leaking. If oil not obviously visible, then plug the line and drive the vehicle. It will shift very firm while you do this but it will not harm anything for a short term. If the plugs remain clean, then the modulator is probably leaking. Third way, apply vacuum with the Mityvac to the modulator and see if it holds the vacuum.

The modulater check is a cheap and easy test to rule it out before you start spending big bucks for head work.

230/8

Tomguy · #10 11-10-2005, 01:19 PM

Ok, you said your retard didn't work, so then you have that line to the distributor disconnected and your timing set to 7°BTDC and NOT 5°ATDC, correct?

BradleyP · #11 11-10-2005, 10:48 PM

Bradley, if your cylinders are all within 5 psi of each other why would a gasket be indicated?

Well, #1 was getting crud on the plug, and the car had some overheating in traffic issues, and after paying $850 for a down payment for a head job and instead having the mechanic give me an $850 tuneup/trans service (which he didn't even replace the filter and gasket) saying my head gasket was fine, and when I got the car back two weeks later, I didn't have it for more than four days before it started overheating again. Took it back and he put a "block tester" on the radiator and as soon as he walked away, it started showing bubbles.

$2398 more later, the car is back.

milterino · #12 11-11-2005, 10:27 PM

Thanks for your replies. Tomguy, I do have it set to 8 before tdc. Bradley, now I understand why the head gasket. I am going to take the car for some junkyard therapy this week, perhaps it will perk up when it sees the junkyard. Milt.

BradleyP · #13 11-11-2005, 11:05 PM

Don't take your german to the junkyard, bring the junkyard to it!

One of my first 108s used to get me back and forth to get parts for itself and I returned once to find the hood star missing. The next saturday, I spotted it on a dodge caravan! (I knew it was mine because of a small, but obvious knick in it. Looked awful on there too. Being 16, I took it back, and not so proudly (ah hell you'd have done it too) removed his valve cores in addition to my star.

psfred · #14 11-12-2005, 09:27 PM

Bad plug wires will often work well enough at idle but loose spark at load due to the nature of spark igntions.

The voltage required to jump the spark gap varies with the pressure of the charge it is going to ignite. At idle, the pressure in the cylinders is less than ambient air pressure, and the voltage required is fairly low. As the throttle is opened, the pressure goes up and more and more voltage is required to jump the gap. If the wires are burned, or have too many internal gaps, both the temperature of the wire and resistance will increase, lowering spark current, and as the voltage goes up, the wire will "leak" current worse.

You probably have weak spark, and it won't ignite the fuel mixture properly at load.

A tuneup is in order -- points ($8, how could you go wrong?), a check of ignition resistor, cap, rotor, and wires if they don't look NEW. Cap is fairly expensive, but you can use the cheaper non-rpm limiter cap just fine.

Make sure the mechanical advance works (if not, you will need to pull the top plate out of the dizzy and clean the internal shaft/hollow shaft, it gets rusty and won't advance).

Peter

milterino · #15 01-19-2006, 10:38 PM

This is a belated reply but the missing problems under load are over. The miss disappeared when I put a set of junkyard wires in. I thnk it was Mark who directed me to electrical.
Another belated reply, some time back I complained of no spark after taking off the distributor. I finally found out what I did wrong. The line from the distributor to the ECU is a coaxial affair and what I did was bend it and broke the interior wire part of the coax cable. Checking it for continuity revealed the problem. Its a very brittle item after 30 years of heat. Milt.