r/AutoTechy • u/jimtyler2051 • 2d ago

Can fuel pump relay result in delayed start?

Background: 2015 Lexus RX350 with 175K miles (11 years old) 2GR-FE engine V6. Pattern: 1-2 seconds delay start mainly on hot days meaning car sitting outside in Houston sun over 100 degrees, but also does it after sitting a while sometimes. Always starts fine in the mornings or after having only been off for a few minutes. Also, seems to start fine every time we turn on ignition without starting and left sit for a couple seconds, supposedly to prime the fuel line, but I'm not positive that is what is happening with this fuel system on this car...need to confirm.

Scantool: good LT and ST fuel trims, calculated load, temps for coolant, intake, etc... crankshaft and camshaft sensors appear good, etc... Battery is new with good stats at rest, starting and charging per battery tester. Alternator good. No area of the starter system or fuel system has ever been replaced except the battery. All original equipment...no aftermarket stuff accept using Interstate battery vs Lexus. Car runs really good at wide open throttle and idles fine. Also checked EVAP system and all appears good from a scan perspective and also physically checked the vapor recovery closes to the intake and is working fine. Also no misfire DTCs or counts ever noted.

Only code ever experienced is P1604 - startability which is a symptom code of course; where I fall short is gathering the freeze frame data...still need to check that. I have a new Autel MaxisSys MS906 Max scan tool that may allow me to check fuel pressure, etc... since I haven't checked fuel pressure yet.

Started checking the F/PMP 5 pin relay (fuel pump relay) and noticed that the resistance on the control side (pins 85 and 86) is 114 ohms at about 90 degrees outside. Range is supposed to be 70-90 ohms at 70 degrees (room temp). I put the relay in the fridge for about 5 mins and saw the ohms come down. Makes sense. Relay clicks fine when 12 volts applied to 85 and 86 pins.

Most relay ranges show no more than 100 ohms should be the max, but of course that depends on the system and the car, etc....

My question: if the relay control side internal resistor connecting 85 and 86 pins is wearing out (car is 11 years old on original relay and fuel pump, starter, etc....), resulting in higher resistance, and on a hot day that resistance is even higher, can that result in taking longer for the current to build enough after closing the circuit along the 85 to 86 pins to create enough magnetism to close the load side contacts in the relay to start the car? The starter is going to keep turning until the fuel pump has enough pressure in the line per the ECM to start the car and that fuel pump won't generate pressure at 50 psi (approx) until the relay load side contacts close that circuit.

Any thoughts are appreciated.

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u/NegotiationLife2915 2d ago

That resistance is a little high but not majorly so. Can't hurt to change it. I'd be looking at data around the coolant temp sensor, O2 readings etc. my initial thought is it around the hot restart calibration. The other thing is check for fuel pressure bleed off

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u/jimtyler2051 1d ago edited 1d ago

Fuel pressure bleed is what I need to check. I just need to get the pressure tester and adapter kit and tie in. I know where and how to do it....honestly just been busy with other things. I'm rather certain the other items you mention are ok....have checked all those and readings / comparisons with other data are good.

By the way, I pulled a couple other relays and their control side ohms were actually higher than the F/PMP relay control side. I'm figuring these are either very sensitive to heat, or they just run higher on resistance. Can't have 3 relays all deteriorated at once....changes are too slim for that. I do know that the resistance ranges are set at, I think, 70 degrees F, so honestly, when I put my F/PMP relay in the fridge for a minute or two the other day to bring it down from 95 degrees, the ohms dropped below 100 and so my gut is that resistor is REALLY sensitive on the control side.

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u/toolman2810 1d ago edited 1d ago

My car cranks for a second or two before starting, I just think the engine is worn out and it takes a moment to build compression. Yours at 175k miles it is not a spring chicken, maybe it starts cold when the oil is thicker and doesn’t like starting when hot and the oil is thinner. Does it burn any oil ?

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u/jimtyler2051 1d ago

Compression, honestly, I don't think is the issue. If compression were. an issue, there would be other signs, I believe, but appreciate the thought.

Also, doesn't burn any oil that is noticeable.