1987wranglerowner Posted October 26, 2015 Share Posted October 26, 2015 HELP!!! I have a 87 wrangler, 2.5. When you test drive it, under acceleration, it gets slower and slower and starts to back fire thru the exhaust. After installing a fuel gauge to the TBI test port and running it inside to the dash, I can ride down the road, accelerate and watch the pressure drop from 15 lbs down to nothing. Based on these findings I replaced the fuel pump, filter, regulator diaphragm and adjusted. I still have the same problem. I ran a wire back to the orange fuel pump wire and tied it in there, connected it to a volt meter and drove the vehicle to see if it was loosing voltage to the fuel pump for any reason. It is not. It maintains 13 to 14 volts at the fuel pump but the fuel pressure will drop while watching both the gauge and meter. What else can stop fuel flow to the injector. I know the injector is part of and is controlled by the ecu system but this seems to be a lack of fuel pressure and seems as thou you would have to cut voltage to the fuel pump to cause my problem, but that is not happening per my testing. By the way, this jeep cranks fine cold or hot, will sit in the garage under acceleration and not miss a beat. Problem is only under a load while driving. HELP PLEASE Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted October 27, 2015 Administrators Share Posted October 27, 2015 1987wranglerowner...Check the return line to the fuel tank. If restricted or pinched, a blockage could cause fuel pressure changes. An uncommon but possible issue would be tank venting. If tank pressure (above the fuel level) were high enough, this could restrict return flow into the tank. Make sure there are no restrictions on the return lines and that the tank's emissions EVAP system functions properly. Check EVAP and canister hose routings. Consider the EVAP canister and also the fuel cap.Moses Quote Link to comment Share on other sites More sharing options...
1987wranglerowner Posted October 27, 2015 Author Share Posted October 27, 2015 Thanks for the response. This morning I cranked it up and pinched the return fuel line under the TB and watched the fuel pressure rise multiple times. I think that tells me there is no restriction on the fuel return line. Then i removed the gas cap and drove it. It still fails. I think that tell me the tank venting is not the problem. I have looked at all the EVAP lines as well as other vacuum lines. Don't see any problems but. If you don't think my return line or gas cap test is effective, please tell me. I keep going back to the thought that its in the electronic fuel injection, something telling the injector to stop delivering fuel but the fuel pressure gauge says its mechanical fuel delivery unless some other electronic device on the TB can stop the mechanical fuel flow. Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted October 28, 2015 Administrators Share Posted October 28, 2015 (edited) 1987wranglerowner...I see where you're going, why you ruled out the clogged or restricted lines. The physical drop in fuel pressure is revealing...Vacuum or EGR could play a role, but I'd be more inclined to see this as a possible electrical ground issue. You can have 13-14 volts but not enough amperage. Try to confirm whether the pump is actually running when the pressure drops.The YJ Wranglers are notorious for ground issues and corroded ground hardware. Check for grounds resistance at the battery to frame, engine block, firewall and even at the fuel pump. Connect an ohmmeter between the pump ground near the pump and the battery negative post. See what kind of resistance you read between these points. This is a DC system that requires ground integrity. You have load on both the ground and hot sides. A lamp test could be revealing at the fuel pump circuit and grounds. A lamp test is good for voltage or amperage drop. The tester is simply a headlamp that illustrates "brightness" under load. You can run a lamp load test on the ground circuit and the hot circuit. Start with a base test on a device that you know works well to get an idea of how bright the lamp should be. Make sure the fuel pump circuit and lamp are of similar load limits to prevent blowing a fuse.You can check vacuum and EGR functions, though this does not sound like that kind of trouble. One item I would not overlook, regardless of its role in this troubleshooting, is the crankshaft position sensor. This is a known "universal" issue around ECU triggering. Also keep in mind that the ECU's trigger for EFI is the ground side. Hot or positive is constant voltage. For EFI fuel supply systems, ground integrity is critical. Check the ECU's ground, too.Let me know if this helps...We'll dig deeper if necessary.Moses Edited October 28, 2015 by Moses Ludel Quote Link to comment Share on other sites More sharing options...
1987wranglerowner Posted November 3, 2015 Author Share Posted November 3, 2015 Good Morning. I guess I have not mentioned it but before I contacted you the first time I had checked the resistance on the manifold temperature sensor and the water temperature sensor. I also removed, cleaned the crank position sensor and tested the resistance. Since your last response I have ran a ground from the fuel pump ground wire at the rear pigtail and grounded it to the frame at the rear, no change. I ran another hot wire from the battery thru a switch to the rear orange fuel pump wire to make sure the pump was never losing power, no change.Even thou the plugs are not fouling, is there anything inside the throttle body that would fail and bleed off fuel pressure under high demand (load)? Is there any gaskets or seals that could go bad and bleed fuel pressure in the throttle body? Quote Link to comment Share on other sites More sharing options...
Speed Posted November 5, 2015 Share Posted November 5, 2015 Could there be a plugged filter screen on the fuel pick-up in the tank?I've had that happen before on another truck and it acted like that. Speed Quote Link to comment Share on other sites More sharing options...
1987wranglerowner Posted November 6, 2015 Author Share Posted November 6, 2015 Hi Speed. When I replaced the fuel pump the screen was cleaned and checked as well as the tank. Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted November 7, 2015 Administrators Share Posted November 7, 2015 Have you tested ground resistance between the battery negative post and the frame, the engine and the body? Curious if your "loop" from pump to frame got back to the battery negative post.There's another issue with continuity testing. This is not the same as insulation resistance tests or the load/amperage carrying capacity of a lead. High mileage Jeep YJ Wrangler grounds are notorious for corrosion and poor connections at the frame-to-body and elsewhere.Have you tested the fuel pump flow volume? Beyond fuel pressure, you also need fuel volume. Triggering power to the pump, see what kind of fuel volume the pump puts out. This can be done by disconnecting the fuel line at the TBI unit and running a fuel hose safely into a suitable can. Power up the pump directly and measure the amount of fuel going into the container. We're looking for volume (pints) of fuel per minute. This could be telling. If fuel volume per minute (or seconds) tests as sufficient, we may need to suspect the ignition performance under load rather than inadequate fuel supply. Or perhaps an alternator issue...Moses Quote Link to comment Share on other sites More sharing options...
1987wranglerowner Posted December 7, 2015 Author Share Posted December 7, 2015 Moses. For you and any others following the thread, I have had to put the problem on the back burner lately. Hope to get back on it this week. Thanks for the advice. I will post future work and results. Quote Link to comment Share on other sites More sharing options...
Speed Posted December 7, 2015 Share Posted December 7, 2015 In addition to enabling one to attend to other stuff,I've discovered stepping away for a week or so will allow my mind to sort things through a bit and take a more "abstract" look at the problem. Often that's when I'll think of something I missed,didn't think of or DID think of but passed off as "not even remotely possible" but suddenly seems much more possible. I have about a dozen projects I rotate through.I work on one until I reach a seemingly insurmountable problem,put it in storage and work on something else until I either finish it or reach a seemingly insurmountable problem,store IT and work on the next one. Most times,the "insurmountable problem",when I come back to it later,isn't so hard to figure out and fix. Speed Quote Link to comment Share on other sites More sharing options...
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