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Silky250 04-21-2019 05:17 PM

78 280 idle mystery
Just finished a head job. Installed a msa header. Deleted egr.
Car started right away. Idled great at 1200, isles smooth at 850 once warmed up. Adjusted idle via screw to 800. All was good. Closed driver door. Car started idling rough and stalled. Wouldn’t restart without throttle application and would barely idle sometimes stalling.
Checked all vacuum hoses. Everything looks good. Car revs up but sounds like it’s misfiring. Checked valve clearance. All good. Car still idles rough.
Close door. Everything smooths out.
Open and close door. Car isles like **** again. Now I can’t get it to idle good again.
Another note. When idling good idle screen works as it should. When idling rough appears to make no change.
When I pull the pcv hose the idle increases but doesn’t smooth out.
I’m at a loss.
Also the car ran fine before the head was pulled. Failed manifold gasket necessitated head job. Nothing else on car was touched

Dave M 04-21-2019 11:06 PM

closing the door is a clue, prob electrical, start checking all the various connectors, starting with the injectors and then sensors. Could also be a flakey ECU, try smacking it with the palm of your hand next time it acts up.

Silky250 04-22-2019 11:41 AM

Checked all connectors. Everything seems fine. Replaced at the injector pigtails a year ago. Pulled the hose going into the air bypass valve. The one that connects to the rubber intake boot. Idle smoothed right out.
If I block the air bypass the engine would almost stall, which is what I’d expect. Oddly enough if I plugged the hose coming from the rubber boot before the AFM I’d get the same result. Checked my air filter. Basically
New. Disconnected the rubber connector up stream of the AFM. Car started and idled fine. Connected air filter rubber boot and everything was fine. So basically everything is assembled as it should be and it’s running /idling great at the moment.
I’m going to try and heat cycle a couple more times and re torque.
Gotta love old Datsun.

Silky250 04-22-2019 02:24 PM

Well, back to square 1. Went and fire down it up this morning. Wouldn’t start without throttle application. Once started would barely idle. 500rpm. Pulled air bypass hose. Idle increased but sounded like ****. Plugged air hose going into bypass valve. Engine stalled. Plugged hose attached to rubber intake boot. Engine stalled.
Pulled pcv hose. Allowed me to plug air bypass hose back together but if I covered the pcv hose the engine would stall.
Removed cover on AFM. It appears any time I plug a hose it causes the AFM to move ever so slightly but just enough to cause a stall.
I’m stumped. Almost seems to be running extremely rich and any increase in the AFM causes a stall.
Guess it’s time to get the FI bible out.

Silky250 04-22-2019 02:53 PM

After writing my last post I got thinking how nothing is adding up.
Going back to my original post that closing the door was messing with thing I went back to the ecu. Cleaned all the contacts. Rapped on the side of the ecu. No change.
Had a spare ecu that came with the car. Plugged that in. Fired right up. Settled into idle. Brought it up to temp and shut down.
Fired up again. Good idle, no hesitation.
Hopefully that was my problem.
Never fails to amaze me how something can go bad sitting in the shop.

Dave M 04-22-2019 05:51 PM

odd but if you think about it maybe vibration from years of use on door may have gotten to it. I have opened them up and the use very high reliability parts, so if anything its prob a cracked solder joint if the ECU is the issue. For fun try installing it again, I have had issues like this where I swapped the ECU, cleared right up, then reinstalled the old one and all is fine. Makes me think it could be a 36 pin connector issue as well.

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