75 450sl craziness

[fluidvp]

So here is run down on this monster. When i bought the car i knew it wasn't running. Apparently the got timing chain jumped when a chunk of chain guide broke off and lodged in the drivers side cam gear. So after rebuilding the entire engine, I am no trying to get it run consistently.

My problem is once the car starts up and runs(idles)for about 5 minutes, the engine starts to shake and from time to time it surges. The engine sound great before that. I have changed and or adjusted the following:

1.cap,rotor,plugs,wires,oil,timing chain,all chain guides,complete engine gasket set, all engine bay fuel lines,all vacuum lines

2. set timing to zero degrees,set dwell to 30 degrees,adjusted fuel pressure to 32 psi,adjusted CO know to mid point

I would appreciate any advice at this point.

[Wodnek]

My 380 had the same characteristics, although it ran great for 15 minutes.

I swapped out the coil. In my case once the coil got heated up from the engine it was not working reliably.

[Graham]

Originally Posted by fluidvp

So here is run down on this monster. When i bought the car i knew it wasn't running. Apparently the got timing chain jumped when a chunk of chain guide broke off and lodged in the drivers side cam gear. So after rebuilding the entire engine, I am no trying to get it run consistently.

My problem is once the car starts up and runs(idles)for about 5 minutes, the engine starts to shake and from time to time it surges. The engine sound great before that. I have changed and or adjusted the following:

1.cap,rotor,plugs,wires,oil,timing chain,all chain guides,complete engine gasket set, all engine bay fuel lines,all vacuum lines

2. set timing to zero degrees,set dwell to 30 degrees,adjusted fuel pressure to 32 psi,adjusted CO know to mid point

I would appreciate any advice at this point.

It sounds like a mixture problem as the car warms up. Could be the coil, but probably not if it happens after 5min.

Have you tried squeezing or blocking off the line from the AAV to the idle valve? After 5 min, the AAV should be partially closed. If it is sticking, it is same as having a vacuum leak. That will cause high rpm. The ECU will cut off fuel over 1500rpm at idle, so then you get surging. I would look at this first.

By setting CO at mid point, do you mean you have a CO meter and measured the CO or did you just set the potentiometer on the ECU to it's mid point? You really can't adjust the mixture on these cars without an exhaust gas analyzer or %CO or AFR meter.

On these cars, the timing should be more advanced. Best to disconnect vac lines and set it at about 30deg BTDC at 3000rpm. This should end up at 5-8 deg BTDC at idle with no vac retard connected. On the '75's, there are two lines to the distributor - one for retard and one for advance. Don't know just how to set that up, but check that the tiny ports in the throttle body are not blocked.

Finally, I would check the manual (available on-line) sections 07.4 and 07.5 and go through the checks of all components. Especially trigger point operation ad TPS. Make sure the TPS reads zero ohms when throttle is against it's stop.

Have fun!

[meltedpanda]

ditto what Graham said and add fuel quality and make sure injectors are spraying correctly , not leaking

[fluidvp]

Originally Posted by Graham

It sounds like a mixture problem as the car warms up. Could be the coil, but probably not if it happens after 5min.

Have you tried squeezing or blocking off the line from the AAV to the idle valve? After 5 min, the AAV should be partially closed. If it is sticking, it is same as having a vacuum leak. That will cause high rpm. The ECU will cut off fuel over 1500rpm at idle, so then you get surging. I would look at this first.

By setting CO at mid point, do you mean you have a CO meter and measured the CO or did you just set the potentiometer on the ECU to it's mid point? You really can't adjust the mixture on these cars without an exhaust gas analyzer or %CO or AFR meter.

On these cars, the timing should be more advanced. Best to disconnect vac lines and set it at about 30deg BTDC at 3000rpm. This should end up at 5-8 deg BTDC at idle with no vac retard connected. On the '75's, there are two lines to the distributor - one for retard and one for advance. Don't know just how to set that up, but check that the tiny ports in the throttle body are not blocked.

Finally, I would check the manual (available on-line) sections 07.4 and 07.5 and go through the checks of all components. Especially trigger point operation ad TPS. Make sure the TPS reads zero ohms when throttle is against it's stop.

Have fun!

I am not exactly sure what you mean by "AAV". As far as CO goes, I was referring to the plastic dial on the side of the ECU.

Good news though. I found than when it runs terrible, that the #1 and or #2(depending on its moment) injectors aren't firing. There is no pulse at the pigtail. So if i am not mistaken, the problem is either in the wiring(didn't find anything wrong, even checked resistance from ECU pigtail and injector pigtails), ECU, or maybe the trigger points and or wiring from them to the ECU.

It seems strange that it is erratic and not consistent though.

[Graham]

Originally Posted by fluidvp

I am not exactly sure what you mean by "AAV". As far as CO goes, I was referring to the plastic dial on the side of the ECU.

Good news though. I found than when it runs terrible, that the #1 and or #2(depending on its moment) injectors aren't firing. There is no pulse at the pigtail. So if i am not mistaken, the problem is either in the wiring(didn't find anything wrong, even checked resistance from ECU pigtail and injector pigtails), ECU, or maybe the trigger points and or wiring from them to the ECU.

It seems strange that it is erratic and not consistent though.

Sounds like a trigger points problem. Sometime just oil on them, sometimes just worn out. Not adjustable (but can be bent a little).

But could also be just a bad ground - check at back of engine where all injector grounds come together and give it a good clean.

BTW - that knob on the ECU is only idle mixture adjustment. Middle of range is a good starting point if the MPS hasn't been messed with - It is the main mixture control. To set either MPS or ECU, you really need a CO analyser, but you can do the idle adjustment just by ear. (MPS=Manifold Pressure Sensor)

[fluidvp]

Great news! While checking out the trigger points and cleaning them, i worked my way back through wiring to the trigger connector on the primary harness side. I found one of the connectors had a nice green corrosion on it. I cleaned the connection and the monster lost 99% of its missing. I appreciate everyone that helped with feedback. Thanks

[fluidvp]

I completely forgot these monsters have firing orders like a v8 FORD. That means it was missing in cylinders 1 and 5. I believe they share the same impulse wire.

[meltedpanda]

the AAV is auxilary air valve. Trigger points fire 4 injectors at a time, so check to make sure it is just not two, if just two may just be bad injectors , check for pulse at wire. Doubt ECU , they are pretty bomb proof

[rowdie]

The other 1% may be the ECU adjustment. I fiddled with mine one click at a time. You have to wait a few seconds between adjustments.

[meltedpanda]

great! Now have fun !