stumbling hard on cold(ish) startup
 

My 84 NA 300ZX's engine stumbles very harshly when it is run for the first time in the morning (stone cold) and you give it gas. The same is also true when it's coldish outside and the engine has enough time to cool off to almost stone cold (like when I get out to the parking lot after a three hour night class and it's 40 degree F outside). Note: it runs just fine at idle - it's only when I give it gas (even just a little bit) that it coughs and stumbles. I have to let the engine idle and warm up for around two minutes before it will operate smoothly, otherwise if I really floor it cold or try and go fast it will run embarrisingly rough or possibly even stall on me. The engine has 200,000 miles on it, and it runs amazingly smooth and wonderful once it is warmed up - it has great power and the gas mileage isn't too bad (around 22 mpg with 90/10% city/highway). Anyway, I think it's a shame for the engine to run so poorly when it's cold - I've read a number of other posts recently on this issue, and it seems to have stumped a lot of other people - so maybe we can finally lick this problem for a lot of people right here and now!

I've had a lot of work done on my Z recently - a lot of new parts on the engine, so maybe we can narrow this down. I have a new Head Temp. sensor, a new O2 sensor, the car was recently timed and had all the belts replaced. My fuel regulator is new also - along with all new gas hoses and injectors up front (the back hoses are original I think, but I don't believe they leak). I just consulted the computer in the car and it tells me I don't have any faults - so it would seem the Crank Angle sensor works properly as does the Mass Air flow meter and my EGR valve. None of these things surprise me, as the car runs great once warm.

Somebody suggested that the problem may be with the Air regulator that bolts into the manifold. How would one go about checking the air regulator? Apparently it needs to remain open until the vehicle warms up... should I just unbolt it, start the engine and observe if it works or not? Any thoughts - as always - welcomed :)

Re: stumbling hard on cold(ish) startup
 

Oh yes... I totally agree with you. I have an old VW with an aircooled engine... and on that thing esp. I always let it warm up a minute or so when it's cool in the morning.

Well hmmmm.... it probably isn't the air regulator then - because my car does idle up and then drop down after a minute or so... all the same I think I'm going to pop it off sometime in the future and take a look at it. This has be baffled...

thanks though!

Re: stumbling hard on cold(ish) startup
 

I was having an intermittent stumbling problem off idle, cold or warm, and seemed to have fixed it. I adjusted the Throttle Position Sensor a small amount and also cleaned the contacts on the electrical connector to the TPS. The connector appeared to be corroded and I think that's what fixed the problem more than the adjustment. I still have a warm idle problem that I'm working on. Since cleaning the TPS connection, it starts and goes into a smooth 1400 RPM idle when cold. As it warms up, it drops to a smooth 800 RPM idle for a minute or two but shortly after starts to idle rough between 1000 to 1200 RPM. I replaced the CHTS and am now looking at the air regulator. The air regulator is open when cold, but I want to make sure it doesn' t leak air through when warm. I cleaned out the air regulator with carburator cleaner and WD-40 to make sure the valve works smoothly, but that didn't seem to help. I've went over the vacuum hoses carefully several times and can't find any suspected leak points. It just acts like an air leak somewhere, but it seems to be sensor related since the problem starts so abruptly after warm up. I'll let you know if I find any solution to the problem.