Bartman Posted October 2, 2007 Author Share Posted October 2, 2007 Do you have extra ground wires going from the engine to the body? Do you have anyway to monitor the fuel pressure while the stumble is happening? I really doubt it a compression issue just because the issue comes and goes. I would still check it just to be sure though. I would try to borrow a PCM from someone local and load up your program into it and see if it goes away. Either that or just buy another one and see if it goes away, if not just resell it on ebay. Guy I have a good ground strap from the engine to the body, but I don't have any way to monitor fuel pressure while the stumble is happening. I have noticed the last couple of times I removed the intake that there wasn't much pressure in my fuel lines (actually it's close to none as I didn't do anything to relieve the pressure and none squirted out when I disconnected it). I have a removable gauge that connects to the Schraeder valve on the fuel rail, but I can't see it when I'm driving. The fuel pump is new as well, but maybe I have a problem with my regulator? Quote Link to comment Share on other sites More sharing options...
80LS1T Posted October 2, 2007 Share Posted October 2, 2007 Are you using the original Datsun wiring to the fuel pump? Maybe there is a large voltage drop from the wiring and when the pump heats up it draws more amperage? I would drive over to harbor freight and get a fuel gauge that you can run up to your windshield and monitor it while this is happening. Your not using the same pump as Tim was though are you? Hmmmmm... still not a bad idea to check the fuel pressure. Maybe its the regulator and not the pump like you said. Guy Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 2, 2007 Author Share Posted October 2, 2007 Are you using the original Datsun wiring to the fuel pump? Maybe there is a large voltage drop from the wiring and when the pump heats up it draws more amperage? I would drive over to harbor freight and get a fuel gauge that you can run up to your windshield and monitor it while this is happening. Your not using the same pump as Tim was though are you? Hmmmmm... still not a bad idea to check the fuel pressure. Maybe its the regulator and not the pump like you said. Guy I'm using an LS1 gas tank and fill bucket with a WALBRO fuel pump installed in it, but I am using the stock Datsun fuel pump wiring. Doing some searching, it looks like I should do a fuel pump wiring upgrade. Here's a link to the fuel pump kit I installed a while back: http://www.fullthrottlespeed.com/itemDesc.asp?CartId={F85C0BDD-F7B3-450B-EVERESTBD76-08444C317B96}&ic=001LS1DFFP I should probably get something like this from the same company: http://www.fullthrottlespeed.com/itemdesc~CartId~{F85C0BDD-F7B3-450B-EVERESTBD76-08444C317B96}~ic~717LS1HOTWIRE~eq~717LS1HOTWIRE%2D1~Tp~.htm I don't see any fuel pressure gauges on Harbor Freight's web site. They have something there I could use? edit: Guy, if you're right on this one...I really owe you big time. Quote Link to comment Share on other sites More sharing options...
wheelman Posted October 3, 2007 Share Posted October 3, 2007 Bart, I'm running an LS1 tank using the stock Datsun fuel pump wiring with no problems. I'd tend to think it's not the pump wiring, that is unless Tim had the exact same wiring issue you have. You got the harness from Tim when you bought the drive train right? Maybe there's an intermittent short in one or more of the sensor wires that is causing the whole problem. Have you ohmed out the harness to make sure you have good consistent conductivity in all the ground, power and sensor wires? I found that while wrapping my harness with e-tape I inadvertently clipped a couple ground wires while removing the tie-wraps I'd originally bundled it with. I had to go back through, ohm out the wires, unwrap the bad sections, fix the cuts and re-wrap it. Was a real pain in the ass, but it was all my fault. Maybe you're fighting a similar issue. Edit: BTW: The fuel pressure bleeds down rather quickly in my system after the pump shuts off when the engine isn't running. I'm not surprised you have no pressure in the lines when you disconnect them. Wheelman Quote Link to comment Share on other sites More sharing options...
80LS1T Posted October 3, 2007 Share Posted October 3, 2007 Well before you go changing anything I would get a fuel pressure gauge that reaches your windshield so you can monitor it while the symptom is present. I do it at work all the time. I just run the gauge under the hood(since the hood opens the other way on cars now) and wire it up with a coat hanger and some tape so I can see it easy and test drive it. If your pressure is dropping off while the problem is present I would say you have a pump problem and not a regulator problem. When your engine is running what is your fuel pressure? What is it with the vaccum hose off the regulator? It should go up when the vaccum hose is off. Usually you wont have a drivabilty concern when a regulator fails. You will just notice bad fuel milage and the smell of gas. I have seen wiring issues on cars come and go just like wheelman said too though. You could ohm out all your wiring but that wont tell you if you have bad wiring unless the wire is open when your checking it. It also wont tell you the condition of the wiring, you could have 1 strand out of 20 going to a sensor and it will ohm out just fine. Have you tried a "wiggle test" on the harness. Just move it around while its running to see if you can duplicate the rough running. If you can't find anything with the wiring or fuel I would look into finding a "new" PCM and load up your program into it. I used to have an extra one but I sold it, didn't think I was ever going to need it. I wish I still had it so that I could send it to you to test... Guy Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 3, 2007 Author Share Posted October 3, 2007 Thanks for all the info. I'm going to install the upgraded fuel pump wiring, because it's relatively inexpensive and probably a good thing to do with my upgraded fuel pump. My wiring harness looks pretty tired with lots of spliced wires and I've been thinking about getting mine reconditioned or buying another one at some point. I'll see how it does after I get the intake installed again, and then do some more testing to see where I'm at. Quote Link to comment Share on other sites More sharing options...
sweetride2go Posted October 3, 2007 Share Posted October 3, 2007 i havnt checked fuel pressure in a while, but it was good last i checked... and sometimes it throws codes, but this last time i got a new one, an 'ignition grounding code' ? or therabouts... ill hav to go trough and check all my grounds... Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 11, 2007 Author Share Posted October 11, 2007 Just wanted to give an update on this issue. I brought my car into my friends shop to have him help determine what's going on. He made some recordings, checked a few things out and made some adjustments for me...and it turns out he did it all for free. He used smoke to find and fix a few vacuum leaks that stilll remained, but it still leaks a little from the shaft in the throttle body. This shouldn't be enough to cause the problems I'm experiencing. The fuel pressure is definitely low and could be causing the problem, so I'm going to pick it up tomorrow morning and install the fuel pump wiring kit I got a few days ago and see how it does after that. The pressure was fine before, but I don't keep the fuel pressure gauge installed on the Shraeder valve because it leaks a little. Maybe the wiring started getting fried over time? He also told me the PCM would give erratic readings while he was tapping on it, which it shouldn't be doing; so I may need to replace that as well even though he doesn't think that's why I'm having the stumbling problem. Quote Link to comment Share on other sites More sharing options...
Danno74Z Posted October 12, 2007 Share Posted October 12, 2007 Bart, When you said the pressure was low, what is the actual reading on the pressure gauge? So you think this is a voltage drop to the fuel pump? Thanks, Danno74z Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 12, 2007 Author Share Posted October 12, 2007 Bart, When you said the pressure was low, what is the actual reading on the pressure gauge? So you think this is a voltage drop to the fuel pump? Thanks, Danno74z My friend told me it's around 31 psi and that it should be between 41 and 47 psi. He also told me that removing the vacuum from the regulator had no effect at idle and that if he momentarily closed off the return line the pressure didn't shoot up. He said that if you momentarily pinch off the return line the pressure should shoot up almost immediately. I'm not 100% sure that the wiring is the culprit, but it is a very likely suspect. Acording to testing by Full Throttle Speed, a change from 13 to 12 volts on a Walbro pump (which is what I''m using) will reduce fuel pump delivery by 20%. http://www.fullthrottlespeed.com/itemdesc.asp?CartId={F539DC6F-167B-4B4D-918CEVEREST-B242C408E265}&ic=717LS1HOTWIRE&eq=&Tp= Quote Link to comment Share on other sites More sharing options...
Danno74Z Posted October 12, 2007 Share Posted October 12, 2007 31PSI - Wow that is low. That level psi would probably get by with a stock LT1 engine but with your mods you probably need to be up at the 47psi level. I know you have 30lb/hr. injectors what size fuel lines are you running? Did your friend also check to make sure the regulator itself is OK? Is the reg the stock unit or an aftermarket? Just maybe you have solved you stumble problem - or getting very close! Danno74Z Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 12, 2007 Author Share Posted October 12, 2007 31PSI - Wow that is low. That level psi would probably get by with a stock LT1 engine but with your mods you probably need to be up at the 47psi level. I know you have 30lb/hr. injectors what size fuel lines are you running? Did your friend also check to make sure the regulator itself is OK? Is the reg the stock unit or an aftermarket? Just maybe you have solved you stumble problem - or getting very close! Danno74Z I'm still using the stock LT1 regulator and 73 240Z lines, but I'm using the stock vent as the return (the stock vent line is larger diameter than the stock return line). I was reading this article and they talk about fuel pressure and flow consistency suffering above 350 horsepower in LT1's with the stock fuel rails because they are plumbed in series. http://www.gmhightechperformance.com/tech/0409htp_lt1_fuel_system_modification_third_gen_use/ So I'll try to get my fuel pressure up to where it needs to be with the new wiring, but more upgrades may be in my future. Maybe an Aeromotive adjustable on-rail regulator with an Auto Meter fuel pressure gauge connected to it would be nice place to start. Plumbing the fuel rails in parallel would be a nice upgrade some day as well. Quote Link to comment Share on other sites More sharing options...
Danno74Z Posted October 12, 2007 Share Posted October 12, 2007 Bart, you mentioned that you are using the stock 73Z fuel lines except swapped the return line. IMO you need to be using some bigger fuel lines in your car. What size lines does Guy use in his LT1? You may have BOTH a low voltage issue and a volume problem. Your car is somewhere around 450HP at the crank. From everything I have read you need at least -6 or -8 lines going to the engine. You will fix the voltage issue but you will run out of pressure and flow as soon as you get on it a little. Guy is using -6 (3/8") hard and flex lines on his engine. If I'm not mistaken the early Z's were 5/16" dia. Danno74Z Quote Link to comment Share on other sites More sharing options...
wheelman Posted October 12, 2007 Share Posted October 12, 2007 Bart, Isn't that fuel pressure issue seen more when the engine is at WOT and is caused by all the fuel being dumpped through the injectors in the first rail so the pressure drops in the second? Are you seeing the stumble off idle, mid-rpm throttle changes, at WOT, during steady state cruising or randomly? Sorry if you've already described this, I didn't want to go back and read the whole thread. Edit: I agree with Danno74Z about the size of your fuel lines. I installed 3/8" lines (feed and return) just so I wouldn't run into a flow problem. I'm only making ~315HP at the crank so your engine will definitely need more fuel than mine. Wheelman Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 12, 2007 Author Share Posted October 12, 2007 Yes, replacing the fuel lines should be on my upgrade list as well. If I'm unable to achieve the desired fuel pressure with the upgraded wiring, it may need to happen now. Ken, the stumbling usually occurs when I'm at lower RPM and then try to accelerate moderately. When the problem occurs the fuel pressure is very low and the system is rapidly jumping in and out of closed loop. There still may be other issues, but without getting the fuel pressure issue resolved we can't move on to see if anything else is going on as well. Quote Link to comment Share on other sites More sharing options...
80LS1T Posted October 13, 2007 Share Posted October 13, 2007 Yeah Im running -6AN supply and return lines. I also had -6AN fittings welded into the back of the fuel rails so I could run my lines right from the areomotive adjustable regulator and back down to the return line.(i'll get pics tonight for ya!) Your fuel pressure should have jumped up about 10psi with the vaccum hose off the regulator. I also agree your fuel pressure is way to low. I run mine at 45psi(if I remember right...). Quote Link to comment Share on other sites More sharing options...
deja Posted October 13, 2007 Share Posted October 13, 2007 I agree that your fuel pressure is low. I'm no expert but my local expert told me the run 3/8" lines both ways. The stock LT1 lines are 3/8" in and 5/16" return. No clue if this is your stumbling problem but it could be. Quote Link to comment Share on other sites More sharing options...
80LS1T Posted October 15, 2007 Share Posted October 15, 2007 Here's my fuel system... Aeromotive adjustable fuel pressure regulator with fuel pressure gauge attached to the front of it. You could put a sending unit where the gauge is and use a electic gauge inside your car if you want to. The back of the fuel rails have -6AN fittings welded in. The passenger rail is the feed line and the drivers is the return. The fuel rails also had some kind of tube running through the middle of them, those are now gone so the rail actually holds more fuel now. I have seen no problems at all since removing them. I am also running a Mallory cartrige style fuel filter. The whole system to and from the tank is -6AN. The hard lines are alluminum, which I ran in the stock location. No extra heatshields either. Guy Quote Link to comment Share on other sites More sharing options...
Danno74Z Posted October 15, 2007 Share Posted October 15, 2007 Guy, Nice pictures of your LT1 fuel system. I had a question about the middle picture. My Camaro fuel rail has one pipe - front and rear that connects the left and right fuel rails. I noticed that yours has twin rear pipes. Did you do this mod or did it come from the factory this way? Danno74Z Quote Link to comment Share on other sites More sharing options...
Bartman Posted October 15, 2007 Author Share Posted October 15, 2007 I installed the upgraded wiring and the difference was immediate and significant. The installation was quick and easy and afterwards the engine fired up quicker and sounded better. I took it out for a drive and for a while thought the problem was completely resolved, but after a while it did start stumbling again...although not nearly as bad as it was before. I need to be able to monitor fuel pressure while I'm driving at least while I'm debugging this issue. Well before you go changing anything I would get a fuel pressure gauge that reaches your windshield so you can monitor it while the symptom is present. I do it at work all the time. I just run the gauge under the hood(since the hood opens the other way on cars now) and wire it up with a coat hanger and some tape so I can see it easy and test drive it. ... I can't find a fuel pressure gauge on the Harbor Freight web site, do they have them at their stores or do they just sell a hose I can use to rig up mine for testing purposes? Guy, thanks for the pictures. I notice you're running the Aeromotive regulator, do you think that's a worthwhile upgrade? It would probably make it easier to install a fuel pressure gauge as well (instead of using the shraeder valve). Its looking like I should upgrade the lines and regulator regardless of whether it's causing my stumbling issue or not. Quote Link to comment Share on other sites More sharing options...
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