LS91owner

My LS1991, after being parked in the sun for a few hours and then driven for about half-hour in hot weather above 80 degrees F, loses acceleration and then has erratic idling / acceleration . After the car cools off for an hour, it can be driven again. No issues during winter driving

Can someone please advise if I should change:

• Fuel Pump Resistor, or
• Fuel Pump Relay, or
• Capacitors on the ECU
• or all of the above

Thank you,

oldskewel

iTrader: (1)

I had a very similar issue on my '91. For me, it ended up being a soft-failing fuel pump.

Here's a post I made a while ago, while figuring things out. If you follow it, you will basically be able to check each of those electrical components. I did all that, confirmed them all good, and identified the fuel pump basically by process of elimination.

https://www.clublexus.com/forums/ls-...ml#post9777959

I found my problem was more likely to happen when the tank was relatively empty (<1/4). I also found that loosening the gas cap to relieve pressure would fix it sometimes. In general though, very tough to diagnose since the soft-failing fuel pump failed weirdly.

More posts in that thread probably give more details and the fuel pump part number.

Most people seem to just start swapping in parts without any testing, which is why I wrote that post to help people avoid that.

LS91owner

Thank you. I will investigate and report back.

deanshark

My 93 used to do the same thing, like you mentioned, only when it was hot outside. My problem turned out being the Air Flow Meter (AFM). There's an Air Intake Temperature (AIT) sensor inside the AFM that goes bad but doesn't throw a code. I unplugged my AFM and the car ran beautiful. I know for a fact it wasn't the ECU because I am using that ECU in my 94 which I'm driving now and it doesn't do it at all.

Sin1UZFE

You mean MAF?

deanshark

Like I said, AFM. I even wrote it out. Air Flow Meter. The car is a 91.

Sin1UZFE

I'm guessing it's different in a 99

oldskewel

iTrader: (1)

Yes, these '91s have AFMs that optically detect and count vortices to measure air flow, and then use a measurement of air temp to account for density variation. A little more detail in this post:
https://www.clublexus.com/forums/ls-...l#post10206613

But either way, every car I know of should run in some sort of decent / safe backup mode when the AFM / MAF connector is inplugged. So it's an easy troubleshooting test in general. If you unplug and the car still runs badly, you know it can't be the AFM since it's not even connected. If you unplug and the problems go away, that suggests the AFM is involved in the problem.

And BTW, just as another idea to keep in mind on this problem, if you look in that thread linked for the AFM info, you'll see the guy's ~similar issue was fixed with a throttle body cleaning and throttle position sensor repair / adjustment.

Sin1UZFE

I wonder if my MAF needs cleaning as my car has a weird starting issue when hot and outside is warm