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.

25 minutes ago, Cruzzar said:

I have pulled the cover and can visually see that the idle points are open when the engine goes lean.

The best way to test the EFI system is with a meter at the ECU connector.  What the computer sees is what matters.  Your fuel pressure readings looked in the ballpark, but 32 at idle suggests low intake vacuum.  The EGR system is often the culprit there.

 

3 minutes ago, Roberts280Z said:

The FSM seems adamant that 36.3 psi is required, in both the discharge pressure check and full-load requirement check.

I think that when he said 32 he meant with the engine running and intake vacuum applied to the FPR.

I have a Ben Watson book on Bosch FI systems, which states in general that the pressure at idle should be 30-35 psi, and should increase by 5-10 psi with higher manifold pressure (i.e. lower vacuum).

I think I recall a situation where my fuel flow was restricted due to the fuel pickup in the tank being partially clogged. I temporarily helped the situation by blowing back through the pump intake line into the tank.

It's kind of funny how automotive mixes up so many different types of measurement, but you can calculate the fuel pressure drop from the intake manifold vacuum, and vice versa.

A good running engine will have about 16 inches of vacuum at idle.  Convert that to pressure and it tells you how much drop the FPR should cause.  About 7.8 psi.  36.3 - 7.8 = 28.5 psi.  14 inches gives 29.4 psi.

32 psi correlates to 8.8 inches of vacuum.  Not very good.  Could be valves, timing, vacuum leaks.

Just more clues to work with.

https://help.summitracing.com/app/answers/detail/a_id/5306/~/engine-vacuum

https://www.onlineconversion.com/pressure.htm

Edited May 22, 20213 yr by Zed Head
wrong number

Zed Head, In regards to the EGR, I have pulled a vacuum on the EGR valve and it holds a vacuum, I have plugged the vacuum port for that hose with still no change in engine performance, I even plugged both vacuum hoses going to the fuel canister with no change. Is it possible that the EGR valve is stuck in the open position and causing a vacuum leak? At idle when warm it pulls 18 inches. of Hg. so I think the pressure regulator is receiving a strong signal. I have not checked the PCV (I don't even know where it is) is it possible that it is stuck? I can pull the hose coming from the top of the valve cover and it will develop a pretty strong vacuum. If there was a large vacuum leak wouldn't it cause a hard start and a lean condition all the way through the rpm range? Engine had good compression (152 low-160 high)

2 minutes ago, Cruzzar said:

At idle when warm it pulls 18 inches. of Hg.

So, you have some inconsistencies, which are probably clues.  I just reread your first post and see the vacuum reading.  That's a good number.  Your fuel pressure at idle should be down around 28 psi.  I've had that on mine.  Maybe you have a bad fuel pressure gauge.  Not uncommon.  Maybe you have bad FPR.  If it's original the diaphragm can stiffen up.

There's a big hose under the intake manifold that connects to the side of the block.  That's the main PCV hose.  They can fail.

Can you describe the problem better.  "It accelerates up to about 2500 rpm and then it falls flat on its face. "  is not very much.  Is it always right at 2500?  Does it make any noises when it fails?  Does it recover immediately at 2300 RPM?  Stuff like that.

The suggestion to watch the pressure gauge while driving is a good one.  Might be something weird like losing pump power.  Under acceleration fuel pressure should stay up around 36 psi.

 

Location of PVC. If in question easy to remove and clean with solvent. If you can shake it and it rattles then it's probably ok.

Another random thought.  If the AFM is old the carbon trace could be bad.  Maybe there's a certain point of the vane travel where the AFM sends bad data to the ECU.  One of the AFM tests is to run it through its travel and watch resistance or voltage.  Can't remember which.

Found it.  It's in the FSM but not the Guidebook.  Page EF-51 in the 76 FSM.

Edit - you can clean the trace with contact cleaner.  Hose it down, move the vane.  See what happens.

Edited May 22, 20213 yr by Zed Head

30 minutes ago, Zed Head said:

Another random thought.  If the AFM is old the carbon trace could be bad.  Maybe there's a certain point of the vane travel where the AFM sends bad data to the ECU.  One of the AFM tests is to run it through its travel and watch resistance or voltage.  Can't remember which.

Found it.  It's in the FSM but not the Guidebook.  Page EF-51 in the 76 FSM.

Edit - you can clean the trace with contact cleaner.  Hose it down, move the vane.  See what happens.

I bought a '76 that wouldn't go beyond idle. Spider web on the afm vane had it stuck. One of my all time lucky breaks. Probably none since then and that was '86, '87.

 

17 hours ago, Cruzzar said:

parked it under a large carport in 1995

Spiders.  Chipmunks.  Mice.  Carports are outside.

Could even be throttle linkage.  Make sure all the moving parts move like they should and that there's nothing blocking air flow.  Check the inlet tube to the air filter too.

Interesting... Pretty much everything sounds like it's working OK, but the engine is starving for fuel once you get above idle.

So to answer some questions: Yes, it is possible for the EGR to be stuck open even if there's no vacuum applied. But that would cause an issue across all operation conditions, especially idle. If the EGR is stuck wide open, I doubt you'd be able to get a steady idle at all. Same for the PCV if it were stuck open.

Sounds like a classic fuel capacity issue, but your fuel pressure up at the rail is OK.

Maybe your fuel injectors are partially clogged? They can pass enough fuel at idle (because the engine just doesn't need that much fuel at idle), but when you're taxing them to pass more fuel than that, they can't. There are little screen filter cups on the inlets to the fuel injectors. Maybe they are partially plugged with crud? Maybe the injectors themselves are plugged, or sticky, or both?

10 hours ago, Cruzzar said:

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.

I'm not sure your conclusions are valid. You're running a tiny bit rich at idle (12.7 - 13.0) which is fine, but just because you can sometimes nurse it up to 3500 doesn't mean you've really got the flow capacity. An engine with no load will "run" with an extremely lean mixture. What's the A/F ratio when you manage to nurse it to 3500?

To me, it sounds like you're a little rich at idle, and as the RPM's go up, you get a little leaner, and a little leaner, and a little leaner, until it's finally so lean that it just won't run at all.

Eureka!!!!!!!!  One of the first things that showed a possible problem when I first ran into this problem was my AFM so I replaced it with a rebuilt one but checked it before installation. I tried increasing the fuel pressure as suggested by Siteunseen with no improvement. I was just about to pull the ECU plug and go over the testing procedures to see if I missed anything but instead I called a friend of mine who has some Z stuff to see if he had an old AFM that I could install and see if it made any difference.       W E L L L L L L L L L ------- I  install this  used, grungy one from a friend  this afternoon and it WORKS!!!  It is not the first time I have ilnstalled a rebuilt item and it didn't work correctly. I don't know what the issue was with the rebuilt one, I had examined the carbon track and it look perfect, it could be the batch of resistors for that portion could have been bad or not the correct resistance because I had already done the FSM's suggested checks on it as a precaution before I installed it. Applying the 12v and looking for reducing voltage as you move the air vane didn't show me anything unusal. Anyway thanks to all who have given me suggestion. I guess if anything I have become a better informed owner which I don't mind. This is no my first rodeo working on car but this issue was driving me nuts because it was not making sense and many of you out there have a tremendous knowledge base with these cars. Will post pictures if I can figure out has to do that when I get it all cleaned up, licensed, insured and on the road.