Jeff G 78 Posted July 30, 2010 Share Posted July 30, 2010 I am out of spit and I only have a few days to figure out and fix the problem with my 260Z 24 Hours of LeMons car or I will have a very long, frustrating race next weekend. I'm asking for and suggestions you have. The biggest issue we have is that it's a track-only car and we cannot prove out anything we try until we are in the race next weekend. Here's the story: Car setup: '74 260Z with 3 screw SUs on a 260 intake and stock exhaust manifold Fuel tank was cleaned and POR15 treated and has two fuel filters that stay clean Engine has good compression SUs were cleaned/rebuilt and SM needles installed. Floats set to spec and they synched at idle and 1500 RPM Pistons drop at same rate and needles move smoothly up and down in choke seats New mechanical fuel pump - electric pump removed New fuel hoses and vent lines All hard lines flushed and checked for blockages Stock magnetic pickup distributor with working vac and mechanical advance Extra heat shielding between exhaust and intake and fuel rail well insulated Performance coil running at full 12+V (no ballast resistor) No EGR or water lines to intake manifolds New MSA radiator with gutted thermostat The sympoms: Car starts immediately and idles perfect as low as 700 rpm Free revs smoothly to 5k+ rpm Runs great on roadcourses for 10-20 minutes with zero issues After 10-20 minutes (depending on air temp), car begins to stumble at 4k rpm. Within a few turns, the car hits a brick wall at 4k and will not rev higher Backfires through exhaust 2-3 times on decel after a stumble Runs worse and worse and within a few turns, car will not accelerate at all Continues to free-rev to 5k+ without missing Sometimes after car cools for 30 minutes or more, it will run well again for 10-20 minutes and other times, it continues to have issue Car always free-revs fine and always restarts immediately and idles smoothly What we've tried: Checked fuel inlets at carbs, checked/replaced filters, floats, etc Symptoms did not change with/without SM needles Plugs are all the same and rather black after a stumble incident Replaced Transistor Ignition Unit immediately following stumble Removed gas cap to rule out tank vent issue Ran car without hood to aid airflow to fuel lines in case of vapor lock Replaced coil, spark plugs, plug wires Our thoughts: We are fairly certain the problem is heat related as is is worse when ambient temp is higher and only happens after car gets fully up to temp We *think* the problem is fuel related since it's only under load. If ignition, would the car free rev OK? We think the problem is in fuel delivery and not in the carbs since the plugs all look alike Our next steps: Install an electric fuel pump in addition to mechanical pump Add fuel pressure gage so we can monitor pressure Recheck mechanical advance in distributor Like I said earlier, the car free-revs fine and it only happens under load, so we have no way of knowing if a fix works or not until we are on a track for 10-20 minutes. Our next opportunity will be DURING our race next weekend. We plan on trying anything and everything we can do in one week, so I'm open to suggestions! Thanks in advance. Quote Link to comment Share on other sites More sharing options...
MONGO510 Posted August 3, 2010 Share Posted August 3, 2010 You may want to check all the wiring harness connectors, I have seen similar symptoms caused by high resistance at a connector. The connection will get hot with time, and the hotter it gets the higher the resistance, causing more heat. The final result is a lack of voltage/current to some vital electrical component. As it cools down it corrects itself. Something to consider. Mongo I am out of spit and I only have a few days to figure out and fix the problem with my 260Z 24 Hours of LeMons car or I will have a very long, frustrating race next weekend. I'm asking for and suggestions you have. The biggest issue we have is that it's a track-only car and we cannot prove out anything we try until we are in the race next weekend. Here's the story: Car setup: '74 260Z with 3 screw SUs on a 260 intake and stock exhaust manifold Fuel tank was cleaned and POR15 treated and has two fuel filters that stay clean Engine has good compression SUs were cleaned/rebuilt and SM needles installed. Floats set to spec and they synched at idle and 1500 RPM Pistons drop at same rate and needles move smoothly up and down in choke seats New mechanical fuel pump - electric pump removed New fuel hoses and vent lines All hard lines flushed and checked for blockages Stock magnetic pickup distributor with working vac and mechanical advance Extra heat shielding between exhaust and intake and fuel rail well insulated Performance coil running at full 12+V (no ballast resistor) No EGR or water lines to intake manifolds New MSA radiator with gutted thermostat The sympoms: Car starts immediately and idles perfect as low as 700 rpm Free revs smoothly to 5k+ rpm Runs great on roadcourses for 10-20 minutes with zero issues After 10-20 minutes (depending on air temp), car begins to stumble at 4k rpm. Within a few turns, the car hits a brick wall at 4k and will not rev higher Backfires through exhaust 2-3 times on decel after a stumble Runs worse and worse and within a few turns, car will not accelerate at all Continues to free-rev to 5k+ without missing Sometimes after car cools for 30 minutes or more, it will run well again for 10-20 minutes and other times, it continues to have issue Car always free-revs fine and always restarts immediately and idles smoothly What we've tried: Checked fuel inlets at carbs, checked/replaced filters, floats, etc Symptoms did not change with/without SM needles Plugs are all the same and rather black after a stumble incident Replaced Transistor Ignition Unit immediately following stumble Removed gas cap to rule out tank vent issue Ran car without hood to aid airflow to fuel lines in case of vapor lock Replaced coil, spark plugs, plug wires Our thoughts: We are fairly certain the problem is heat related as is is worse when ambient temp is higher and only happens after car gets fully up to temp We *think* the problem is fuel related since it's only under load. If ignition, would the car free rev OK? We think the problem is in fuel delivery and not in the carbs since the plugs all look alike Our next steps: Install an electric fuel pump in addition to mechanical pump Add fuel pressure gage so we can monitor pressure Recheck mechanical advance in distributor Like I said earlier, the car free-revs fine and it only happens under load, so we have no way of knowing if a fix works or not until we are on a track for 10-20 minutes. Our next opportunity will be DURING our race next weekend. We plan on trying anything and everything we can do in one week, so I'm open to suggestions! Thanks in advance. Quote Link to comment Share on other sites More sharing options...
Derek Posted August 3, 2010 Share Posted August 3, 2010 (edited) I had fuel starvation problems and tried everything. I ended up doing the "turn the drain plug into an outlet" mod and it solved my problems. I'm thinking I may have had a few pin holes in the suction tube in the tank. Derek Oh and a long time ago (like in the late 80's) I had a similar problem. There was a piece of old tape in the tank. occasionally it would suck up into the feed line. the car would starve and die. Then after sitting for a while the tape would drop out of the tube and I would go on my merry way. Drove me nuts! Edited August 3, 2010 by Derek Quote Link to comment Share on other sites More sharing options...
Zmanco Posted August 3, 2010 Share Posted August 3, 2010 We *think* the problem is fuel related since it's only under load. If ignition, would the car free rev OK? While I haven't experienced this with an electronic distributor, I don't see why a weak spark wouldn't still allow the engine to free rev and yet wouldn't be strong enough for proper ignition of the air/fuel under load. So I don't think you can rule out the ignition. If I've got a good picture of the ignition system, the only thing you haven't changed is the distributor. Do you have access to mechanical points version? You might swap over to it as I'm guessing you're only concerned with the car running for the weekend so the benefits of electronic ignition (better cold starting etc) don't really matter. Don't forget to add the ballast resistor back in, or at least never leave the ignition on when it's not running. The only other thing I can think of is to make sure you have the proper insulator between the mechanical fuel pump and the head as that transfers a lot of heat to the pump and hence the fuel. But the fact that you ran it without a hood and still had the problem makes me doubtful it's a fuel temp issue. Quote Link to comment Share on other sites More sharing options...
Jeff 78 280z Posted September 28, 2010 Share Posted September 28, 2010 Hey. I am very interested in this subject. I have a 78 280z 5 speed with a similar problem. When I start the car it idles great and you can drive the car normaly up to 4000rpms. Then it hits a wall. If you free rev the engine it will only over come 4000rpm's if you feather the gas pedal in neutral. When you floor it the engine hits the 4k rpm wall in neutral. After much head scratching and fuel pressure testing. I have thought that my fuel hose between the tank and the pump was collapseing. So I replaced it. And nope still the same issue. I guess my next step is to unhook the fuel line before the fuel rail and place it in a clear container. Then run the fuel pump and see if I find any bubbles. That would mean my pick up tube has a hole in it. And its sucking air. I suppose I could remove the hose from the tank and place that into a container of fuel to be sure its the pick up that is clogged and or sucking air. I agree its a frustrating problem and if you find a solution please post it as I will post any solution I find. I don't think its an ignition problem cause like I said if I feather the gas pedal in neutral the engine will rev well past 4000rpms. PS. I'm not trying to highjack your thread. Just thought we have a similar problem maybe we can help each other. Quote Link to comment Share on other sites More sharing options...
Noddle Posted September 29, 2010 Share Posted September 29, 2010 (edited) Hey. I am very interested in this subject. I have a 78 280z 5 speed with a similar problem. When I start the car it idles great and you can drive the car normaly up to 4000rpms. Then it hits a wall. If you free rev the engine it will only over come 4000rpm's if you feather the gas pedal in neutral. When you floor it the engine hits the 4k rpm wall in neutral. After much head scratching and fuel pressure testing. I have thought that my fuel hose between the tank and the pump was collapseing. So I replaced it. And nope still the same issue. I guess my next step is to unhook the fuel line before the fuel rail and place it in a clear container. Then run the fuel pump and see if I find any bubbles. That would mean my pick up tube has a hole in it. And its sucking air. I suppose I could remove the hose from the tank and place that into a container of fuel to be sure its the pick up that is clogged and or sucking air. I agree its a frustrating problem and if you find a solution please post it as I will post any solution I find. I don't think its an ignition problem cause like I said if I feather the gas pedal in neutral the engine will rev well past 4000rpms. PS. I'm not trying to highjack your thread. Just thought we have a similar problem maybe we can help each other. I had a similar issue, it turned out to be a electrical issue, only happened after I washed the engine, I cant remember if it was the TPS, or the AFM connector had water in it. I just read this. http://forums.hybridz.org/index.php/topic/95316-braaps-l6-efi-induction-advice-and-tips/ cut.... a) The idle circuit is a richer mixture and will only allow the engine to rev up to 3200 RPM, then as the RPMS drop, the EFI kicks back on at 2800 RPM. This is a VERY violent rev limiter! Many of you have found this after washing your engine bay and the TPS connector gets wet. This is incorporated for high RPM, when you drop the throttle to closed under deceleration conditions, the ECU shuts off fuel delivery to the engine completely for emissions. If you have a very free flowing exhaust, and depending on how rich your AFR’s are to begin with or BCDD removed, you will notice a slight to moderate â€POP†in the exhaust as the fuel is turned back on as the RPMS drop down through 2800 RPM, but only at closed throttle during decel. cut.. Nigel Edited September 29, 2010 by Noddle Quote Link to comment Share on other sites More sharing options...
BLOZ UP Posted November 2, 2010 Share Posted November 2, 2010 I'm having a similar issue on a '77. I've had the tank "cleaned", replace the filter, and blown out the lines. There's still rust in the tank. When accelerating the car hits a wall at 4k in all cases. The more you drive it the lower and lower the fuel pressure goes under load. Will normally idle and rev fine in neutral. However, fuel pump has now been making weird noises and fuel pressure goes up if you knock it with a mallet (or tire iron...). Just recently started hesitating in neutral. So, the fuel pump is dying, I'm ordering a replacement. I've also installed a clear carb filter before the pump inlet, and it sure is collecting a lot of small rust particles. Not sure if something larger is clogging the inlet in the tank. If the fuel pump solves the problems I will be relieved, but I have a feeling I should take my tank back out and on to someone who can guarantee I won't have anything coming out of the tank except gasoline. The 4k RPM thing, at least from an EFI perspective, sounds like a fuel pump flow issue. Either by the pump itself and/or a clogged inlet. Quote Link to comment Share on other sites More sharing options...
bjhines Posted November 2, 2010 Share Posted November 2, 2010 I would lose the later factory dizzy and go with an early model with the Pertronix pickup installed. There is 45 feet of wire feeding power to your coil on the early model Zs. It goes back and forth twice from the dash to the coil. Look into eliminating the extra back and forth runs of wire. Quote Link to comment Share on other sites More sharing options...
4a15 Posted April 11, 2011 Share Posted April 11, 2011 Been there - done that. Sounds like your stock ignition control module in the distributor can't keep up with the aftermarket ignition coil. The aftermarket coil pulls more amperage and causes the control module to overheat. An overheating module will cause weird timing over 3-4k RPM (backfiring, missing, seems like running out of gas). A hot day does not help the situation. Try a stock coil with a resister and I bet your problem goes away. Or upgrade to an aftermarket Ignition control module, they can handle damn near anything. Of course your current ignition control module may just be taking a dump. Good luck. Quote Link to comment Share on other sites More sharing options...
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