Engine leans out over 2400 rpm

[Cruzzar]

First post here but have been a spectator for about 4 months trying to absorb as much information as possible. I recently obtained a 1975 Datsun 280Z from my neighbor who purchased the car new as an anniversary present for his wife. He owned the local Datsun dealership at the time so all maintenance was done by his dealership. She drove the car until it had about 106,000 miles at which time she got a new car with an automatic transmission (this was a 4 speed car). The car has had a pampered life. She always garaged it and took good care of it. He put a non op with the DMV, pulled the plugs and oiled the cylinders, drained all of the fuel out of the tank and parked it under a large carport in 1995.

I purchased it late in Dec. of 2020 and have been slowly working to get it back on the road. I have run into an acceleration problem that I hope someone can help me to diagnose.  Since I know from reading past posts , members will want to know details so here goes. 

I have replaced all of the injectors (corroded terminals and age) and their electrical plugs. New electrical plugs on the water temp. sender as well as tested its ohms  output when cold and hot.  New gasket with sealant on the cold start injector as well as a new electrical plug. New spark plugs, wires, rotor and cap. Verified distributor advances when a vacuum is pulled. Pulled the gas tank (it was clean with not rust) replaced all of the fuel lines by the tank as well as in the engine compartment. Verified the strainer was clean, replaced the fuel pump (gaskets dried out so much it leaked like a sieve) and installed a new fuel filter in the engine compartment. I replace all of the vacuum lines, pulled every plug that I could find to clean them.  Pulled the pan to replace a leaking gasket (had a very small amount of sludge in the bottom) refilled the oil and a new filter. Pulled all the plugs oiled the cylinders, cranked it over until the oil came out of the cam/lifter oiler on top. New plugs, new valve cover gasket, turned it over and it fired up. Let it come up to temp. reset the timing to 10 degrees at 800 rpm as per factory specs (I have the factory service manual). Thought this was really going well until I tried to drive it. It accelerates up to about 2500 rpm and then it falls flat on its face. 

Pulled the AFM and started checking the pins, all looked good. Went thu the Service manual ECU checks, all looked good. Checked vacuum at idle when warm 18 in. of Hg. Pulled the plugs and checked compression- lowest 152, highest 160. 

Installed a air/fuel gauge on exhaust right below the manifold and found it idles at about 12.7 and 13.0 when warm. Try to drive it and it goes so lean the engine stops and resumes when back to idle. Checked fuel pressure and static is puts out 36, when running it sets solidly at 32. I know the FSM states 36 psi but is 32 that out of wack?  Started looking for a vacuum leak and I thought my problem was found when I found a leaking brake booster so I plugged the port at the manifold and took another ride but no change. 

I opened the AFM and loosened the spring tension about 5 gear teeth and still no change. If I bring the rpms up very, very slowly I can get it to rpm beyond 3500 but  not when driving. I don't think it is an electrical (distributor) issue because of the ability to rpm over 3500. I've checked and cleaned the points on the throttle switch. I've watched the position of the idle enrichment contact and it opens right off idle and the full throttle contact is not making contact when the engine goes lean.

Any suggestions or recommendations on what else I can check would be immensely appreciated.

[Patcon]

I am not the resident efi expert but I'm sure one will chime in.

You seem to have hot a lot of the trouble areas already.

If it were me I would rig up a fuel pressure gauge I could see from the drivers seat. I suspect your pressure is dropping off.

If the pressure holds I would pull the injectors and do a flow test into cups

[siteunseen]

I think the throttle switch comes off idle contact about 2500? It may need adjusting or cleaning of the contacts.

https://www.atlanticz.ca/zclub/techtips/tps/index.html

[Cruzzar]

I have had the fuel pressure gauge hooked up as I increased the rpms and it holds a solid 32 psi until it falls on its face at which time the psi goes up. I could do a flow test but at times I can get the rpms up to 3500 if I advance very slowly indicating to me that the injectors have the flow capacity and the a/f ratio is rich enough to maintain combustion. It is like I need to increase the the size of the accelerator jet in a carb. to make the transition from the primarys to the secondaries.

[Zed Head]

11 hours ago, Cruzzar said:

I opened the AFM and loosened the spring tension about 5 gear teeth and still no change.

I would set the AFM back to where it was.  

What happens when you increase RPM under no load?  When the engine "falls on its face" what is the tachometer needle doing?  The tach needle is a good way to see what's happening with your spark.

Does it pop back through the intake and buck when it loses power?  If so, try the coolant sensor tweak.  It's better than the AFM adjustment.

https://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

[Roberts280Z]

My experience with this situation on this and also with Bosch K points to fuel pressure or possibly clogged screens or filters.

[240260280]

I agree with @siteunseen

 

Quick, easy troubleshooting test: Unplug the Throttle Valve Switch and see if problem goes away.

 

 

Edited May 22, 2021 by 240260280

[Cruzzar]

Siteunseen, I have cleaned both set of points and checked their continuity, both are good. From what i have read the off idle should come off sooner than 2500. I have pulled the cover and can visually see that the idle points are open when the engine goes lean. I could bend the arm on the go rich (second set of points) and have it engage sooner but this was a running car in good condition when it was parked.

[Cruzzar]

Roberts280z, when I replaced the fuel pump I checked the inlet screen and it was clean ( it was a brand new Nissan pump for the 280Z). I put a new fuel filter and blew out all steel lines before replacing the hoses. The fuel pressure check valve will lose about 15 psi overnight. The car will start up in a very short period of cranking and idles very well. Initially on start up it will be about 12.1 and within a short time go to 12.9 to 13. This a/f ratio will change with the changing of the gear in the AFM . The weather here is warm so I don't think the cold start injector is even operating. I have had it unplugged and it doesn't start up any different

[siteunseen]

Another shade tree trick is to clamp the return fuel line to increase fuel pressure and see if your problem improves. Best of luck and we'd like to see some pics of this car. Sounds like a super nice '75.

[Cruzzar]

240260280, I unplugged the switch  yesterday and problem still exists. I am wondering if it could be an ignition advance problem. I put my light on it and at 2500 rpms I can get about 25 degrees of advance. It it hard for me to get a good reading above that rpm because of the engine wanting to slow down.

[Cruzzar]

Siteunseen, great idea, I have a tube clamping visegrip pliers and will try that, that should eliminate a lower than normal fuel pressure issue. This is why forums like this are soo good for car owners.