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Odd hesitation problem, efi guru's look....


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So, I've been trying to chase down the hesitation problem on my 77' 280z before the next auto-x. The car drives fine when you first start it and pulls clean all the way to red line. After say a minute or two of driving the car begins to hesitate around 3500 rpm and up. If I pull over, shut the car off for about 30 seconds and restart the hesitation is gone for another minute or two of driving. So far I've checked the coolant temp sensor and afm according to the shop manual. Both are within their operating specifications. I am leaning towards an ignition problem, possibly coil?

I checked the base timing, at idle with the vacuum advance disconnected I saw about 13 degrees, which is good in my book. The car hesitates weather or not the vacuum advance is connected or disconnected. I also have the cold start valve, egr and air regulator removed.

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My '76 did something similar when I first got it. It ran fine when cold, then began missing above 4,000 rpm or so. I thought it was electrical, and I tried everything to fix it without any luck. I finally tracked my problem down with a fuel pressure gauge. I had plenty of pressure at idle, but the pressure dropped the higher the RPMs went. The hesitation would start when the fuel pressure dropped to 25 or so PSI. I had a clogged strainer in the inlet of the fuel pump. I removed the strainer (actually the little bastard wouldn't come out, so I broke it out) and put a filter inbetween the fuel tank and the fuel pump... problem solved.

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A little update. Switched out ecu with spare, same issue. Fresh fuel filter even though the last one didn't look to old, same problem. Tested the coil, got 1 ohm of resistance on the primary and 11k ohms on the secondary. I couldn't find any specs for the stock coil, but these seem reasonable. Fuel pump is a new walbro and the old pump shows no signs of crud. When I drained the tank no rust or crud was found.

 

Also, just cycling the key will get rid of the hesitation for a short period. So say I'm driving along and it starts hesitating at 4k rpm, turn the key off and immediately back on. The hesitation is gone for about 30 seconds to a minute.

 

I'm almost getting to the point where I want to just scrap the stock efi and run mega squirt. But I know its just one simple little thing causing it.

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How about the ignition modual?

Not a bad suggestion by Woldson me thinks

Is the ignition module stock? My experience with the stock ignition module was that when it was starting to go south it would heat up when you revved the car up high RPMs and go wacky.

When you say hesitation, do you mean when you step down to accellerate? Or like a surge at cruise? explain more the conditions of the hesitation. Also, is this stock fuel injection system? Have you checked/replaced all the vacumn hoses? Cold start, EGR, ect. removed. Where do you live, what are the ambient temps?

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Hesitation is only under load. So it can be 1/4 throttle or full throttle and it still does it. If your cruising and holding it at say 3k rpm it will cruise fine until you try to accelerate. If you put in the clutch it rev's up freely, turn the car off and back on. The hesitation is gone for a minute or so. It is stock efi. I will check into the ignition module as I was thinking the same thing, just haven't had the time to check it yet. No vacuum leaks, new afm boot, freshly cleaned fuel injectors, fresh fuel pump, fresh fuel filter etc.

Thanks for the suggestions. I am going check the fuel pressure this week too just to narrow things down. But I really have a feeling its ignition related.

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How about the Air flow sensor? maybe the ECU is running on a pre prgrammed fuel table until it gets all the info it needs from the sensors then once its switches over its getting bad readings from the MAF . Could even be a dirty track on the TPS?

 

this may help:

http://www.atlanticz.ca/zclub/techtips/efisystem/280zfuelinjectionbook.pdf

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  • 3 weeks later...

Did anyone ever solve this problem?? Cause I'm experiencing the same thing. Mine problem is all of the above but then occasionally it just wont start after its warmed up. I'll be out driving and it'll just die. Then I cant start it for a few minutes. Then as if magic i'll try and start it and it'll run like nothing ever happened. I have another AFM being sent to me and I wonder if thats it.

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  • 8 months later...
  • 2 months later...
  • 2 months later...

I have read the EFI bible but some of it is over my head. I dont have the money to repleace every part until i finally find the problem. My 75 280z is doing the same thing. The car had been running really rich and the idle would go up to 2000rpm when warm. It failed emissions misserably. I changed the plugs and removed the air regulator. It had been getting oil blown through it, i cant find any other threads about this problem. The engine has 190k miles so im wondering if it isnt getting reverse flow from the rocker cover sending oil through the air hose. Anyways i replaced the air regulator and the car ran great, halved the emissions but failed again (only just). I decided to run a bottle of guarranteed to pass through it. It had the same hesitation problem as mentioned above. Most noticable on the high way at 2800-3500rpm. This cleared after half a tank was burned and ran beautifully. Then i put regular (instead of premium) and it quit on me 6 times. After waiting 15 minutes (and it cooling down) it would start first try and off id go again. Finally it wouldnt start and i had it towed home. Eventually it would start again but run sooooo rich. I completely removed the air regulator and it started fine but reved up to about 2000rpm (no smoke). I cleaned the air regulator and reinstalled it. Ran soooo rich it died again. I could just put another air regulator on but no doubt ill have the same problem next month. Any ideas? I wanna do the JTR conversion but i dont have the money atm.

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