Bad Miss (sometimes)

[Darrell Woods]

My '75 450SL has a 71/72 4.5 V8 from a 300SEL (#117.981). Late winter through the spring, it had a bad miss (seemed like I lost 4 cylinders). It was intermittent, and some days would miss very little, and others leave me wondering if I'd get home. Did'nt seem effected by humidity/rain, but now that it's warming up, the miss is gone. Once temps reached mid-sixties or so, I noticed the miss wasn't there. When it did happen, it would cut in and out in a way that almost seemed electrical (i.e., no running a little rough, and then a lot...it was either all or nothing). Any thoughts on what might be temp related? Some sort of temp sensor that effects vacuum, which would cause problems with the D-Jetronic Fuel System? Thanks for any insight.

[psfred]

Bad points, (this thing still has a breaker type ignition), bad secondary ignition parts (cap, rotor, wires), loose connection for the transistor box, bad transistor box, bad fuel pump relay (no fuel pressure = no power), or bad fuel pump or connections to pump.

Unfortunately, you will have to determine which one is bad before you can fix it....

Next time it starts to miss, and you are in a position to get out of the car, crawl under the rear left side and listen to or put your hand on the fuel pump (it's fairly obvious) and see if it is running constantly and smoothly. The professional alternative is to connect a fuel pressure gauge and check for constant pressure. Note that a bad pressure regulator might do this too, but only when the engine is cold.

Otherwise, an ignition analyzer will help with the ignition stuff.

If you do have ignition problems related to the transistor switching unit, I'd personally switch to a Crane or Pertronix breakerless ignition and eliminate the points.

Another quick fix may be to clean the trigger points in the distrubutor. They signal the FI to inject, and if dirty will cause the computer to fail to injecti fuel intermittantly. If you have a surge on warmup, this is a likely cause. The worst case here is a worn out distributor lower seal and bushing, that will need to be fixed -- oil will leak up constantly, requiring regular trigger point cleaning until the sela and bushing are fixed.

Peter