74 450 SEL Will Not Start - PeachParts Mercedes-Benz Forum

WINTS · #1 11-15-2006, 07:04 AM

Hi All,

My 74 450 SEL had been running fine. It seemed a little underpowered, but started on the button and warm idle was fine. Cold idle, when first starting is around 1300-1400rpm.
After a 10 minute journey the other day I slowed to a stop to enter a complex and then as I speed up again to only 10mph the revs died and it backfired. The revs came back, it ran normal and I parked. The car started and ran without fault for the rest of the day, which was another 20 miles or so.

The next morning it started first time and after driving for 10 mins, about 5-6 miles, the revs just died again and now the car will not start. No backfire this time.

Basic ignition problem? Do I start at the plugs and work back..?

Tomguy your 2 cents appreciated!

alabbasi · #2 11-15-2006, 10:14 AM

Have you checked the system?
Ignition is easy to check, just pull a plug wire out and place it near the top of the engine (rocker cover) if you see an arc when you crank the engine, then your getting a spark.

If it was running and then died, it could be a fuel problem.

david s poole · #3 11-15-2006, 12:26 PM

ok old d jetronic system.first make sure that there is no oil between the points under the distrib cap.in the bottom of the distrib there is a set of trigger points that controls the fuel[two clys at a time therefore 4 sets of points,they also suffer from oil contamination[you will need to set engine on tdc and remove distrib to access]these are the most common intermittent faults with this old fi. system.

Tomguy · #4 11-15-2006, 09:50 PM

Quote:

Originally Posted by WINTS

the revs died and it backfired. The revs came back, it ran normal and I parked.

Backfired how? Through the intake? That's almost always a sign of ignition issues. Backfiring through the exhaust could be too much fuel or no spark, a little harder to pinpoint sometimes.

Pull plugs on opposite ends of the firing order spectrum (1 and 6, or 5 and 3, etc) - these both try to fire at the opposite times AND are controlled by different parts of the trigger points. I'm ruling out dirty trigger points for now because the 4.5 WILL start AND run on 4 cylinders, maybe even 3 (though it'll run like garbage, it WILL run).

Here's what I'd do first:
1) Verify spark to the #1 and #6 cyls (spark to both wires, both strong - note that to check this, the spark usually won't jump from the inside of the boot where the connector to the plug is to the outside, so checking with a spare plug is ideal)
- If no spark to at least 1, investigate ignition
- If spark to both, investigate #5 and #3 - if no spark, it's ignition, if spark, it's fuel
2) Pull the plugs, see if they're wet. If so, you're flooded - pull off the trigger point harness (from the distributor) and the cold start valve plug, and crank with the pedal floored. It will unflood the engine. If they're bone dry, no gas. If they're damp but not drenched, you have a good fuel mixture, and it should run if your timing is set properly.
3) Verify engine timing (if the distributor came loose or your advance stuck, starting might be impossible; check this after verifying your other systems, you can set to TDC on #1 while cranking, then adjust from there after it's running)

WINTS · #5 11-16-2006, 08:19 PM

Thanks for the reply's guys.

Tom, the backfire was thru' the exhaust. I have fuel delivery. My distributor base is very dirty and oily. There is a build up of general crud there and I am sure this could be part of the problem along with the wires that attach there, which look like they are being held in place with some help from some nasty looking old black tape.....

BTW...When looking at the top of the distributor, after removing the cap I only have one set of points. Shouldn't there be two?

Tomguy · #6 11-16-2006, 09:17 PM

No, one set of ignition points on a 4.5 - the 6.3 had two sets I believe, but not the 4.5!

The wires at the bottom of the distributor are for the trigger points - Please, if the plug is oily, pull it out and clean it with contact cleaner and plug it back in to see if it helps at all. If your backfire was through the exhaust, it's possible either:
1) Too rich to run in the engine, hence pouring raw gas into the exhaust (poof)
2) No spark in the cylinder, hence pouring raw gas into the exhaust (poof)

I'd suggest at this point a full removal of the distributor. Take out the trigger points and clean them off. Use a TINY TINY amount of grease on the lobes inside the distributor during reassembly. New ignition points. Reassemble the distributor and retime it during cranking.