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Gremlin (solved!)

General Tuning Discussion

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Hey everyone!

So as the title says been chasing this little **** STUPID CAR LOL.

Lexus IS300 with 1JZ VVTI.

Dual AEM 320LPH pumps in the Squash Performance dual pump system but only using 1 pump. -8 AN supply. -6 AN return using 91 octane fuel.

Brand new! Tomei FPR with gauge car off pump on, pressure set to 45psi.

precision injectors 525cc. installed in stock fuel rail.

D-jetro POWER FC using Pro software from APEXI YES the PRO PRO software

First, we found that 1 pump from the dual pump setup was dead and not working. So we were afraid to have some back feeding through the dead pump since the feed is fed in the same chamber, so we removed the dead pump and blocked off that port and left the 1 working pump which should have no problem feeding the fuel necessary at 17psi of turbo boost.

Begin the tuning process, first everything is going smoothly. Im steady state tuning all the way up to 4000rpm nice and smooth afr are spot on, no trouble, YET

Once I begin ramp running the vehicle 2k to 6k rpm soon as the car begins to see 4800-5000 rpm the air fuel begins to go lean!!! I add more fuel to that section. Perform another pull bam same thing air fuel begins to go excessively lean, abort ramp run. lay the 2 runs together and same exact air fuel numbers and same exact point the fuel begins to go excessively lean, even with the fuel map being added.

SOO, I have a buddy stand by engine and stare at the fuel pressure gauge while I perform ramp run again. Soon as the rpm gets to 4800 again it begins to lean out at a fast rate, I immediately abort run and he says the fuel pressure went from 60psi it dropped to about 42psi. Also he noticed the fuel pressure gauge would fluctuate from 58psi to 60psi as the pull was being performed, was not a steady pressure reading.

Shut the car down and began scratching my head. Pulled fuel pump assembly out for inspection, all looks well all wiring is done correctly, pump sock is clean and not touching anything, all is tight secured properly no problems found. no leaks found from tank to stock fuel dampener, fuel rail to injectors no visual leaks.

after the check was all said and done. I performed another ramp run test. Only this time pressure held fine, air fuel levels were excessively rich to 6k :). Buddy said that the gauge was steady at 60psi the whole ramp run. Began performing the tuning process BUT!!! after 30minutes or so again the damn fuel pressure began dropping same o thing happening again!!!!

Its almost as if the pump is losing efficiency after sometime of it running. Anyone seen a pump act funny when hot? is that even possible? Ive heard of pumps just stop working when hot, but never seen this type of problem before.

Only thing I have not done is changed the fuel pump to a new one just yet. owner purchased a new one now but thats our last resort. Why is this pump working efficiently before 4800 rpm but after that its like NOPE!

Did I miss something? What would you guys check before that I have not ?

Thanks for your time guys!

Danny G.

We had this problem with a Bosch 044 pump -- the inlet adapter used had too many threads and thus was restricting the flow into the fuel pump. Only a problem at high flow rates. You saw that when you asked for more flow (increased the fuel request in the table), it got worse.

You have a fuel pump volume problem. Could be a fuel filter, the pump, or a fitting. Try testing the pump straight into a fuel jug. Run it for say 30 seconds or 1 minute and figure out what the maximum flow rate is. How does that compare to the pump specs?

Since it got better after opening the fuel tank. Any chance that the tank isn't vented (or if vented with a check valve that's installed backwards.

Good Luck!

What would the tank vent do?

I will try that flow rate into a jug with this pump with it cold and with the pump hot from running the car for a while and see the difference why not.


The tank vent allows air to fill the space vacated as the fuel is removed. I had a clogged vent on one of my race cars, and it would develop a miss after about 10 minutes of hard running. If it starts to happen again, try removing the gas cap and see if that cures the lean condition...

Do you happen to have a fuel temp sensor? High fuel temp can cause cavitation.

I found these videos helpful.

@ducie54. No actually didn't think about taking the temp of the fuel. It's winter in California right now and the car is on a dyno wouldn't think the fuel would be getting that hot. But I will check out those videos for more information. Thanks. Will also try taking temperature readings of the fuel around the pump. Thanks for the tip!!

Alright everyone. Tomorrow I will be trying to tune this car again. We will have a spare fuel pump with us to install if none of the tips and hints resolve this dumb issue. Will post up the results. Or what I find out with this issue.

Thanks everyone.

Are you able to log/monitor the supply current to the pump? this should remain a fairly constant Amperage, with a small step variance when the engine comes under load and the FPR tries to maintain the correct pressure differential across the injector by raising the base pressure. If it is a pump heat related issue, then there will be a steady rise in the current draw as the pump heats and losses efficiency. If it is a more sudden increase in the current draw, then you are looking at an issue that is, for some reason, loading the pump more to operate, this may be a blockage in the inlet, too great a vacuum in the tank from non functioning vents, a restriction upstream of the pump but before the pressure sensor.

@Blackrex thanks for your input but sadly this ecu is not capable of that information. We ended up checking for any clogs. All was good.

Good news. We swapped the fuel pump to a new walbro 410lph. And success!! The pump did great held pressure all throughout the Rev range. For the entire tune session with no hiccups or anything I'm so happy. This is one problem for the books to be honest. Those dang AEM pumps! To many bad reviews and I'm one to have come across this issue. But you live and learn. I want to thank everyone that helped me out. Really appreciate it guys.

Please take a look at my graph and comment. I don't mind tell it like it is. Good, bad, could be better (if so where) 91 octane 15psi.

It's really my first real tune from start to finish. Dynapack steady state tuning. Full throttle pulls 11.3 afr. Will be used for drifting.

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