Pig Rich on Cold start

[Chickenman]

I've had this problem for some time now. 1976 280Z. Car has always been pig rich on cold start. Once it warms up it's fine. This happened with the old engine with the Stock Camshaft and the new engine with the 280 degree cam.

 

Things I've checked and changed.

 

1: New ECU CTS and new harness connector to sensor. Have used DeOxit on all connections.

 

2: Cold start valve has been completely disconnected. Doesn't seem to make much of a difference if any.

 

3: Plugs, cap, rotor and wires all new.

 

4: New thermostat.

 

5: Just re-adjusted dizzy phasing. PO had it set up so that ignition timing was " Retarded " 6 degrees on cold start-up instead of being advanced 6 degrees. Setting it right has helped warm - up... but the excessive cold richness still remains. And it seems to be intermittent now...

 

6: New AAR Valve. When it behaves, cold RPM goes to 1,500 then slowly settles to a nice steady1,000 RPM

 

AFM seems to be fine. We set it up on a 4 gas analyzer and it's spot on. It had not been previously dicked with. 

 

After about 3 - 5 minutes of running, engine smooths right out and excessive richness goes away. Any suggestions as to what I might have missed? 

 

 Car pulls HARD to 6,500+ and I've recorded 28 MPG on highway. Just this rich start-up issue.

Edited June 7, 2015 by Chickenman

[Captain Obvious]

I got nothing that wouldn't still be an issue after it warms up.

 

CTS and AFM issues wouldn't go away once warm.

Cold start valve could be leaking even though it's electrically disconnected, but that wouldn't go away once warm either.

AAR valve never does anything to affect mixture, it's idle speed only.

 

Maybe the richness curve vs temp is just steeper than you're used to on newer vehicles? I mean, it certainly isn't as well controlled was it can be with todays vehicles. Maybe it's operating as intended and you're just expecting different?

 

Maybe it IS still running rich after it warms up, but you don't notice it as much because it does a better job of completely burning the charge?

 

Just tossing out ideas...

[rossiz]

fwiw, my 78 always ran like a turd in warm-up mode... would barely rev over 2k until it warmed up a bit.

the entire efi system had been replaced - every single sensor, hose, wire, injector, even the ecu and afm were new, every contact cleaned... everything.

 

i thought it was just how the L6 ran cold, until i switched to su's and surprisingly it runs really well with chokes on till it warms up now. perhaps just the way the efi was calibrated on these cars? to be clear: i have no experience with other z's so my impression is only from this one.

[siteunseen]

Have you tried routing your AAR hoses like the later '78s?

That made a noticeable difference in warm up on my car. It pretty much did away with the 1200 rpm when cold.

[Chickenman]

Siteunseen: Yes I have AAR valve rerouted as per 1978. Doesn't really seem to make make any difference to start up other than keep the AAR Valve cleaner. .

 

Capt Obvious: Cold start valve leaking? Now there's something I haven't thought of. I'll plug off the line and see if that helps. It's definitely not something " normal ". Wonder if it could be an injector sticking when cold? I'll have to get them serviced.

 

Took the car on a long cruise today up to Whistler. Car ran beautiful. Tons of power and Torque. Had a bad " Hot Start " problem today at a 20 minute " Rest Stop ", Rare for this car, but it was stinking Hot out ( 95+ ) and I was running the AC all the way back, till we stopped. Had to open hood, poor water over Injector lines and wait about 15 minutes till I could get it to start. Gonna have a good hard look at that Cold Start Injector ".

 

Keep the ideas coming gang.TIA.

Edited June 8, 2015 by Chickenman

[Captain Obvious]

Well I'm not expecting that you'll find anything wrong with the cold start injector, but it is "possible". Prudent to check, but I'm not expecting a revelation.

 

So what else.. Injector that sticks open some when cold? Again, it's possible, but I wouldn't put any money on it.

 

Have you checked the validity of the signal coming from the incoming air temp sensor? Been a while since I went through the theory, but IIRC, the air temp sensor has less of an impact than the water temp sensor, but could still be causing some kind of issue if it's not accurate. To check it, keep in mind that the temp-resistance curves for both the water and air temp sensors are the same, so when the engine is cold (and the coolant is the same temp as the surrounding air) you should get about the same resistances from both of them.

 

Maybe you've got something wrong with the fuel pressure regulator that gets better when the engine warms up. For example, a leak in the vacuum control hose to the regulator could cause fuel pressure to be too high. And if that leak somehow minimizes as the engine warms up...

 

Like I said, just tossing stuff out there for discussion.