skullivan64 Posted February 12, 2015 Share Posted February 12, 2015 Ok, so I have a 1975 280z 2+2 that runs but not well. I have replaces the fuel pump, FPR, and the fuel filter and am running 91 octane with some octane additive. When I'm pushing the throttle it has a huge hesitation in reving up but if I pinch the fuel return line and give it throttle it runs like a champ, no problem at all. I'm stumped... can anyone help me?? Thanks Quote Link to comment Share on other sites More sharing options...
NewZed Posted February 12, 2015 Share Posted February 12, 2015 What do you think is happening when you pinch the return line? Use some logic. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted February 12, 2015 Share Posted February 12, 2015 Make sure your ignition system is working well. Check your plug gap, ignition wires and connections, and verify the ignition timing is right. All you are doing when you plug the FPR return is boosting the fuel pressure beyond the setpoint of the regulator and making your engine run rich. Either you have too little ignition advance or possibly a plugged air filter. Or the air flow meter is not properly reporting the air flow. Does the engine ping when it's hesitating? Quote Link to comment Share on other sites More sharing options...
skullivan64 Posted February 13, 2015 Author Share Posted February 13, 2015 There is no pinging but when I push the throttle it will rev up extremely slow (if at all) and it will make poping noises like its backfireing but the poping comes from the intake, not the exhaust. I should probably check pressures with a gauge, I just haven't gotten around to it yet... another thing to mention, even though the pump is brand new it is very loud (humming that changes with voltage) like it can't keep up with the pressure needed. And when I say that I pinch that return line and build the pressure back up, it runs exactly like it should be running. Responsive to throttle with no hesitation, no backfire, very clean sounding. Thats why I'm thinking its solely a pressure issue but I'm not positive Quote Link to comment Share on other sites More sharing options...
skullivan64 Posted February 13, 2015 Author Share Posted February 13, 2015 Oh, also the air filter is not plugged because I dont have it on at the moment and I've checked the air flow meter components and done resistance checks on it as well. As far as I can tell, no problems with AFM. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted February 13, 2015 Share Posted February 13, 2015 There is no pinging but when I push the throttle it will rev up extremely slow (if at all) and it will make poping noises like its backfireing but the poping comes from the intake, not the exhaust. I should probably check pressures with a gauge, I just haven't gotten around to it yet... another thing to mention, even though the pump is brand new it is very loud (humming that changes with voltage) like it can't keep up with the pressure needed. And when I say that I pinch that return line and build the pressure back up, it runs exactly like it should be running. Responsive to throttle with no hesitation, no backfire, very clean sounding. Thats why I'm thinking its solely a pressure issue but I'm not positive Those symptoms exactly match what my '78 did when I bought it, back in 1990. Turned out that the PO had installed the wrong injectors, too small. When I replaced them with a new set, it ran like it should have. You need to get hooked up with some good tools to diagnose your potential lean condition. An exhaust analyzer and fuel pressure gauge will help you figure out where the issue is. Maybe you have a sticky injector, I doubt your fuel pressure is at fault, but you need a gauge to rule it out. Quote Link to comment Share on other sites More sharing options...
NewZed Posted February 13, 2015 Share Posted February 13, 2015 Extra resistance on the coolant temperature circuit to the ECU would probably "fix" it. A potentiometer allows tuneability. I have two "lean" AFM's in the garage. The use of a potentiometer is described around the internet. Here's a good link, bottom of the first post - http://forums.hybridz.org/topic/95316-braaps-l6-efi-induction-advice-and-tips/ Quote Link to comment Share on other sites More sharing options...
TimZ Posted February 14, 2015 Share Posted February 14, 2015 Extra resistance on the coolant temperature circuit to the ECU would probably "fix" it. A potentiometer allows tuneability. I have two "lean" AFM's in the garage. The use of a potentiometer is described around the internet. Here's a good link, bottom of the first post - http://forums.hybridz.org/topic/95316-braaps-l6-efi-induction-advice-and-tips/ That's true, but do your due diligence and check the fuel pressure first. If the fuel pressure is ok it's possible that your injectors need cleaning - I'd do that before applying any band-aids. Also, any idea as to whether we are talking about a stock engine or not? The factory EFI was really good for it's day but it did not handle changes to the engine's breathing capabilities gracefully at all. Aftermarket "hot" cams generally ran like crap and required band-aids like NewZed mentioned above to run at all. Quote Link to comment Share on other sites More sharing options...
bradyzq Posted February 16, 2015 Share Posted February 16, 2015 Another thing to consider is that the fuel pump is not getting good voltage. Check this while the engine is running, and preferably while it is exhibiting the symtoms you describe. IF original, the wiring in these cars is OLD. Quote Link to comment Share on other sites More sharing options...
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