threepwood Posted August 19, 2016 Share Posted August 19, 2016 Hi, I have a 1992 LS400. Driving on superhighway this past weekend at 65mph the car started bucking and surging. So bad I had to get off the highway and take local streets home. On the way home I would shift it into neutral and rpm would drop to near zero then i would pump the gas and rpm would jump up to 3000 rpm. Then I would let off to get the rpm down to 1500 and put it back in gear. it would run fine for a few hundred feet then the surging would start again. I might have said it was acting more like bad gas (water) than anything else but I'm pretty sure it wasn't. Unscrewed gas cap and tank was pressurized. Started trip again and it kept doing the same thing. Let it sit overnight, thinking I maybe lost another ignition coil (even though I replaced both the last time 1 failed).. But that time the exhaust turned cherry red from the unburnt fuel. This time it did not. Next morning I started it up, ran it for 2 hours and it ran perfectly with no issues??? I checked coils anyway and they are good. Talked to a mechanic friend and he thinks it's a bad fuel pump relay. Is this a possibility? Any other thoughts? Many thanks for the help in advance. Quote Link to comment Share on other sites More sharing options...
CELSI0R Posted August 20, 2016 Share Posted August 20, 2016 Any check engine light? Have you checked for any codes coming from the OBD-I diagnostics port? You can perform a bypass to test the relay. Here is an article that can help you out; even though it's for another car, the concept and application is the same. Quote Link to comment Share on other sites More sharing options...
Exhaustgases Posted August 20, 2016 Share Posted August 20, 2016 The fuel pump relay is not involved with the fuel pump until a lot of power is demanded, other wise the fuel pump resistor is where the pump gets its power. So the resistor is suspect. Start with a new fuel filter, and then a fuel system pressure check would be nice too. It could be bad gas and could be a fuel pump going out, it could be the ECU capacitor problem. And it could be electrical signal problems. Quote Link to comment Share on other sites More sharing options...
threepwood Posted August 20, 2016 Author Share Posted August 20, 2016 Well, I pulled the fuel pump relay, took it apart, looked at contacts and they were well worn, even felt sticky. I cleaned them up with some very fine sand paper, put it back in and now the car is doing the surging ALL the time. This relay is rather expensive, like $80. So I take the cover off again and I wedge the contact to the energized position. Car stats and seems to run just fine. check voltage on contacts to ground with ignition on, 12 volts. Ignition off 0 volts. check voltage on relay coil. ignition off, 0 volts. ignition on 0 volts. If I understand the book correctly, deenergized is directly to fuel pump and energized is thru the resistor to the fuel pump. This would indicate to me the resistor and pump are fine and the contacts deenergize are shot. Do you guys agree? So are they any dire consequences to leaving the wedge in until i get a new relay. My mechanic friend says I will kill the battery, but I do not read any voltage on the contacts until I start the car. Ok - more - I just went out and wedged the contacts in the deenergized position to see what happens. Car starts and rev's just fine. So if you had to wedge it until you get a new relay, which way would be better? Quote Link to comment Share on other sites More sharing options...
CELSI0R Posted August 20, 2016 Share Posted August 20, 2016 The car was not designed to be run with a bypass. It is important to note that a bypass is only meant for testing purposes of a relay. I do not suggest continuing to run the car with this temporary fix. Given how worn the relay is, I would replace it with a new one to ensure proper function. Quote Link to comment Share on other sites More sharing options...
threepwood Posted August 21, 2016 Author Share Posted August 21, 2016 I have ordered one from Rock auto ($15) - hope it fits. Fortunately I don't need to run the car, in fact it doesn't have plates anymore. It started acting up on the way home from the honda dealer, who wouldn't give me anything for it in a trade. I took the plates off it and put them on my new honda accord v6. But I did not want to sell it to anybody knowing it could do this to them randomly at any time. Dealer said even though it only has 150k miles it's only worth $500 in trade and it would cost him $400 to send it to auction so he wouldn't even take it. very disappointing. Quote Link to comment Share on other sites More sharing options...
Exhaustgases Posted August 21, 2016 Share Posted August 21, 2016 You can by pass relay and all, at the data port on the engine B+ jumped to FP. That is how to test fuel pressure and limp home if the relay and resistor go bad. The resistor is probably not good. Quote Link to comment Share on other sites More sharing options...
threepwood Posted August 22, 2016 Author Share Posted August 22, 2016 Thanks, that essentially what i did by wedging the the relay in the deenergized position. I forced the no resistor side to be in service all the time. I put an ohmmeter across the resistor and it was within spec. Quote Link to comment Share on other sites More sharing options...
Exhaustgases Posted August 24, 2016 Share Posted August 24, 2016 Is it in good shape and what color and what state? What options does it have? Quote Link to comment Share on other sites More sharing options...
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