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suddenly rough idle when warm
https://slantsix.org/forum/viewtopic.php?t=61654		 Page 2 of 2
Author:  SlantSixDan [ Fri Mar 09, 2018 10:26 am ]
Post subject:  Re: suddenly rough idle when warm
Quote:
why did the lash get so tight?
Valve recession and/or loose adjustors working their way tightward with vibration.
Quote:
I did an upper-cylinder treatment with SeaFoam (spray it in through the spark plug holes, let it sit for a few days, go out for an Italian tuneup). The goal was to free any rings that might have been stuck, but possibly a side effect was to clean off some buildup that was preventing the valves from rising to their full height.
That's not it. The engine would have run very poorly, if at all, if crud on the valves were preventing them closing fully. And Sea Foam won't have taken much of any crud off the valves.
Author:  rich006 [ Fri Mar 09, 2018 12:33 pm ]
Post subject:  Re: suddenly rough idle when warm
The 1974 head has hardened seats, right? Shouldn't those resist recession?

As for the adjusters tightening on their own, they are hard enough to turn that I wouldn't expect that to happen. I have to use a little body weight to turn them with one hand and a relatively short 3/8 box wrench. Admittedly I'm not a big or especially strong guy. Also, I noted that they each seemed to need almost exactly the same amount of loosening.

Maybe the adjustment was never right, and the warm weather masked the problem until fall/winter.

Anyway, I'll adjust them again and report back.
Author:  69a100 [ Fri Mar 09, 2018 4:23 pm ]
Post subject:  Re: suddenly rough idle when warm
I've had no problems using Remflex gaskets on my exhaust. Good Luck http://catalog.remflex.com/category_s/87.htm
Author:  SlantSixDan [ Fri Mar 09, 2018 7:42 pm ]
Post subject:  Re: suddenly rough idle when warm
Quote:
The 1974 head has hardened seats, right? Shouldn't those resist recession?
The 1974 head is 44 years (that is almost half a century) old. The hardened seats resist recession, yes, but not forever. They slow it down, not eliminate it. The hardening is very shallow, about the depth of a spark plug gap (0.03" to 0.04", that is three to four hundredths of an inch) so it's easy to grind through it on a routine valve grind job.
Quote:
Maybe the adjustment was never right
This happens.
Quote:
and the warm weather masked the problem until fall/winter.
This doesn't happen.
Author:  rich006 [ Sat Apr 14, 2018 10:05 am ]
Post subject:  Re: suddenly rough idle when warm
I started this thread 5 months ago but I'm still troubleshooting. I thought I had it fixed but I didn't. (I now know that the problem is much less noticeable on warm days, which is when I was working on the car. I'd make an adjustment, it would run better, and then on the next cold day it would be bad again.)

Symptoms:
- On cold morning start, it starts OK but after a few seconds becomes very rough and stalls when I try to accelerate; it runs a bit better as the engine warms. On warm days it starts fine and runs fine while driving (except for lots of ping going uphill) until coming to a stop light.
- After driving, it idles OK for a few seconds to several minutes (much longer in warm weather, not at all in very cold weather), then begins stumbling intermittently (hot day) or continuously (cold day).
- When accelerating from the rough idle condition, it stays rough until it suddenly smooths out as rpm reaches 1500 or so (it smooths out sooner on warm days).
- Sometimes it's either better or worse after shutting off the engine for a while (I believe this helps on cold days and hurts on warm days).

Things I've done/checked:
- Timing set to 0 degrees (factory setting) with vacuum advance disconnected; previously timing was advanced 8 degrees, but I retarded it back to the factory setting trying to reduce ping
- Valves adjusted to 0.010", 0.020" with engine running and thoroughly warm
- Checked manifold vacuum: at idle, needle jiggles from 18.8-19.0"; at about 2000 rpm it's steady around 16"; when throttle is suddenly released from about 2000 rpm it jumps temporarily to 24.5"
- Spark plugs looked good when the problem first developed; today some were blackened and one had significant buildup (see photo)
- Compression test (engine warm): 130, 125, 140, 135, 135, 130
*note: I had a shop do a leak-down test last year and the leak-down was 20-40%; they said the rings are shot
- Filled tank with 93 octane (no noticeable change from this except the ping might be slightly reduced)
- Tried removing air cleaner cover to rule out a clogged air filter (nothing changed with the air cleaner open)
- New 190 degree thermostat installed not long ago; engine temp reads 190 on an infrared thermometer

Image
Author:  rich006 [ Sat Apr 14, 2018 11:09 am ]
Post subject:  Re: suddenly rough idle when warm
Quote:
- Checked manifold vacuum: at idle, needle jiggles from 18.8-19.0"; at about 2000 rpm it's steady around 16"; when throttle is suddenly released from about 2000 rpm it jumps temporarily to 24.5"
Clarification: the vacuum readings I quoted above are from when the idle is behaving normally, in neutral at 750 rpm. When it's rough, the vacuum drops to about 17" in neutral, or 13 to 15 in gear. I don't know if low vacuum is the cause of the rough running, or the effect of it (rpm is around 500 in gear in the rough-running condition).
Author:  ceej [ Sun Apr 15, 2018 1:16 pm ]
Post subject:  Re: suddenly rough idle when warm
EGR sticking open/signal from vacuum amplifier due to wear or wrong vacuum signal when engine warms up and the thermal relay opens maybe?

CJ
Author:  rich006 [ Fri Apr 20, 2018 8:52 am ]
Post subject:  Re: suddenly rough idle when warm
Replacing the spark plug wires solved the problem including the pinging. I think the cold start problem was actually my adjustable choke control being out of adjustment (the set screw was too loose and it slipped).
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