1991 miata stalls only when cold and is difficult to restart.
 

Hey guys, I tried this over at m.net but I think ill get better answers here.

I have a late n/a 1991 1.6l. It had this issue before when it had low compression and I rebuilt it and that issue has been resolved.

So what it does is on overnight cold start up it idles perfectly around 1200-1300. I back out of the driveway and about a 1/2 mile down the road the engine sounds like it drops a cylinder, almost like a boxter engine. When I come to a stop it will stall. The only way to prevent this is to apply half throttle to keep the revs over 3k.

Once it stalls it takes about 45 seconds of cranking and feathering the throttle to get it to restart and then it is fine again, It wont do this again until the next cold start.

I have checked for vacuum leaks and I am confident I do not have one.
I have replaced the plugs and wires (ngk coppers)
I have replaced the idle valve
I have replaced the coil pack with a brand new one
I have replaced the thermo switch on the back of the head with a known good used one and then 2 successive new ones
I have reset timing to stock and then to 14 with no impact on this issue,
It has done this through multiple tanks of gas
I have seafoamed the block
I have checked for codes and found none

The only things I can think of is that its either a bad injector, bad cas, or bad ecu.

It doesn't smell rich when Im trying to restart the engine, and it doesn't smoke so far as I can tell when it does restart. I feel like it fuel related weather its flooding or getting no fuel I cant determine, but I feel like the issue is in the fuel system.

Any thoughts?

are you running the stock injectors? whats it run like while its warm? try cleaning the power connectors for the injectors.

It is on stock injectors, When its warm it runs great, but when cold after a 1/2 mile of driving its like it loses a cylinder or 2. If you don't come to a stop it will clear up after 2 miles or so.
The fuel harness was replaced and it did not correct the issue, I also used dielectric grease on all connections upon reassembly.
I would imagine if it was the injectors it would do this at all times. this isnt a random occurrence either its on every cold start.

Im totally baffled, Is this possibly a failure mode for the cas? I feel like its flooding which is why I think its so hard to restart. and it requires feathering the throttle. I cant imagine its a spark issue.

Do you let it warm up before driving?

no but that shouldnt be necessary to prevent stalling. I would understand reduced performance, but not misfiring and stalling 1/2 mile into the drive.

Replace the CAS and injectors and see what happens.

I am not sure why, but I wouldn't think injectors since it works fine the rest of the time. Have you tried leaving it running and seeing if it stalls out after a bit? In otherwords, try to figure out if driving it has something to do with this or if its coincidental.

Also make sure you got the ground strap hooked up to the motor. It's the one that bolts on the shelf underneath your brake master.

It does have the ground strap, and it will not do it just idling it must be driving, I am starting to think that it might be over pressurization of the hla resulting in the valves not fully closing. Im running 30 weight oil since the rebuild I just reached 1000 miles so its going back on 5w30 tomorrow. and prior to the rebuild I was running pretty thick oil because I had compression loss through the rings and I was trying to bandaid it until my rx7 was back on the road.
I feel like the oil pumps relief valve may not be able to relieve enough pressure because of the thick oil. What do you think?

^ I think not.

definitely not. I run 15w50 (or a similar w50) in the summer and 5w40 in the winter and have zero problems of the sort.

As unlikely as it may seem, since going to 5w30 the issue has not resurfaced, granted its been 3 days but still. strait 30 is substantially heavier than 5w30 or even 15w50 when cold, after the engine warms that viscosity changes, but when cold there is quite a big difference.

I didn't realize you were running a straight w30. doh!

Dreyko, mine does exactly the same thing. Have replaced plugs and wires. That smoothed out the idle but no affect on problem. I pull the car out of warm garage and there is NO problem. Car runs smooth and idles at stop with no problems. I've tried sitting and letting it warm up but it's as if the gas is shut completely off. Crank for 30 seconds to get it started and seems to start better if you mash the pedal to the floor and keep it there. If carberated I'd think it was flooded but don't think that's the problem. BAFFLING.

only a year late with this post. id issue a warning if i want on my phone.

Hi guys, I'm new to the forum. But I'm having the same issues. Has anyone come to a definite solution to this. I've reset the idle, replaced fuel filter, full service, and even replaced battery. Is oil the issue here? From memory I think I used 10-40w oil when service was done.