'72 W108 SEL 4.5 stalling in gear - but starts right back up.

[Treozen]

Hello folks -


I have an odd new issue, and I am not certain where to start the diagnosis:

1. The car starts, idles, and drives out fine at first. (sometimes the first start after sitting for weeks is a little stumbly for a second or two)
2. Once warm, I find the idle when in gear is not even / misses slightly at a red light, and eventually the car has a little trouble pulling away smoothly - once at speed though, seems fine
3. When back home after a run, the car started stalling out in gear - like when I moved from park to reverse. Stall was somewhat random, but repeated.
4. In park and warm after the run, the engine sounds and runs normally, but, did at least once have a random sputter, then go back to running fine. I had it idling in park for a good 10 minutes, just one random sputter.
5. After stalling, the car will restart without hesitation.

So - The basic issue seem to be that once warmed up, the idle becomes unstable and eventually the car will stall out in gear (if sitting still, doesn't seem to stall when moving) - note that there is no sputtering or warning, it just quits like you turned the key off. Fuel pressure is also good.



I think I may hear a vacuum leak from somewhere behind the engine, so I'm going to investigate that, but otherwise there are no obvious issues that stand out as a cause. Fuel pressure is good, spark is obviously present, and it seems to be load-dependent i.e. no stalling in park, but put it in reverse and it stalled multiple times. This is a new issue, with no major changes to any system save installing a fuel pressure gauge on the fuel rail.



I'd appreciate any thoughts on this.

Allan.

[BWhitmore]

A vacuum leak will cause a rough idle and stalling. A good running engine will have 18-21 inches of vacuum at idle.

[Graham]

Originally Posted by Treozen

Hello folks -


I have an odd new issue, and I am not certain where to start the diagnosis:

I'd appreciate any thoughts on this.

Allan.

Could be many things.

Do you have a tach on that car? The idle rpms will drop as the car warms up. Down at <500rpm, the car will cut out. Maybe try increasing rpms a touch?

Otherwise one thing at a time. Sticky AAV. Points, ignition/coil/distributor wiring (big and small!) Fuel pressure supply. Temperature sensor connections. etc. etc.

By the way, our Djet vacuum is usually more like 16" Hg when timing is set to about 5BTDC at idle. May have once got it to 18"!

[Treozen]

Originally Posted by BWhitmore

A vacuum leak will cause a rough idle and stalling. A good running engine will have 18-21 inches of vacuum at idle.

I plan to investigate it, but I am not sure its a new thing - but it'll be the first thing I look at - I am not sure there is a leak, but sort of sounds like it.

Originally Posted by Graham

Could be many things.

Do you have a tach on that car? The idle rpms will drop as the car warms up. Down at <500rpm, the car will cut out. Maybe try increasing rpms a touch?

Otherwise one thing at a time. Sticky AAV. Points, ignition/coil/distributor wiring (big and small!) Fuel pressure supply. Temperature sensor connections. etc. etc.

By the way, our Djet vacuum is usually more like 16" Hg when timing is set to about 5BTDC at idle. May have once got it to 18"!

No tach, but I can hook up the digital timing light and get RPMs that way. I don't think its low idle, but I'll certainly check it.



The AAV is a suspect I suppose - it wasn't working at all when I got the car, so I took it off, took it apart and "refurbished" it - I got it working again and the car responded well when I replaced it, but that doesn't mean its working as completely as it should or that my repair has held up. So I'll figure out a test for that. Might just get a new one, if they make them.



The temp sensor connections - Will need to research those. I plan to replace the coil and look at the points. I previously made a trigger point tester as well for a different car, but that is sort of a drastic step since If I screw it up, well.......good luck finding a set of trigger points!

[Graham]

AAVs likely no longer made or ridiculously expensive if found. Trigger point repair kits were available for a while, but maybe not now. They can be adjusted. Hopefully not your problem.