Loo-10-ant

I would change the Engine Coolant Temp (ECT) sensor 1st. It is the easiest and least expensive, most NOTICEABLE fix ever. You can get it from Autozone. $22...mine was $17 I think...anyway it fixed all my issues with it running terrible. See my posts ...it was the end of the road for me...and it worked!
-BM-

BrittonJ85

Whenever yo have engine sputter problem while the car isn't moving. You should 1st try checking spark, fuel and vacuum flow.....

Spark:
Check your spark plugs, wires and routers. I recommended you replace all if they are cheap, bad or if you do not know when was the last time you had them replaced....

Note: I had ordered/installed cheap spark plugs and they fouled out a week after (no lie)

Fuel: 1st check your fuel filter to see if there is any sign of residue like junk which can cause a problem too but I recommend that you just replace the fuel filter rather wasting time. 2nd have a shop check to see if your fuel pump is operating correctly.

Vacuum: Check your vacuum lines to see if there are any wear & tear. Replace vacuum lines if needed.

In conclusion, what helped me with my sputtering issue was to replace all spark plugs, wires and replacing the fuel pump. I notice that once I replaced the spark plugs and wires that the car stop shaking and sputtering.... And once I changed the fuel filter, the car stop cutting off if I gave it too much gas or decelerating.

Good luck!

488civ10

Replaced my fuel pump about 4 months ago with the Denso unit from RockAuto. No problems so far. I've probably only gone through 4 tanks of gas, and it hasn't gotten truly hot yet out in SoCal. At least on a couple of those tanks, I ran the fuel level pretty low, and experienced zero problems. Also, my fuel economy is better and more consistent. Before, I would get some awful mileage readings, but only intermittently. I did the engine coolant temperature sensor a month prior to replacing the fuel pump, and it didn't seem to do much (although it needed to be replaced... it pretty much broke in half after I got it out). Anyway, so far so good, I feel like the fuel pump was the culprit. Hopefully I've put this one to bed for my car.

Interestingly, the Denso pump sounds different, I can definitely hear it, not intrusive or anything like that, but a slight buzz.

Boyscout

iTrader: (3)

I also upgraded my fuel pump. i went with Walbro, but i have also noticed a great improvement in gas mileage and no more bogging (fingers crossed)

IndyLexus

So for those of us who still have the hot weather, long trip stalling and bucking problem has replacing the fuel pump solved the problem?

My 1991 LS400 with 225,900 miles, that I have owned for 22 years, runs great all winter but continues to have this problem every summer and still baffles every mechanic who has tried to fix it. No codes, can't duplicate problem, etc.

Or is it necessary to also replace the fuel filter, fuel ECU, charcoal canister, and other valves in the fuel delivery system?

Do you still have big pressure build up in the gas tank when you remove the cap?

A long term udate would be greatly appreciated.

Boyscout

iTrader: (3)

this is what i have replaced and the 'bogging' has been reduced substantially:

- fuel pump (i used walbro replacement)
- fuel filter (part store brand, not OEM)
- fuel pressure regulator, feed side (part store brand, not OEM)
- coils (part store brand, not OEM)
- plugs
- plug wires
- i also use "Purple Ice" coolant additive to help it run a bit cooler in the summer

488civ10

In my case, the pump was the culprit. None of my previous actions helped the symptoms, though the ect might help overall, now that the pump is new. With the original pump, even when it wasn't hot out, the mileage was poor. Let us know when you get a new pump installed.

IndyLexus

Thanks, I will try the fuel pump and engine coolant temperature sensor first. It may be a few weeks before I can get to it. I intend to do it myself and I will post my results.

There are several fuel pumps available for my 1991. Any recommendations as to which one? I am not making any performance mods. Does it make any sense to use a higher flow pump like the Walbro.? Or might that affect fuel milage?

I may get the one with the longest warranty.

Thanks again for responding. I had to have the car towed last week when it was 84 degrees in Southern Indiana. I was trying to limp home with the sputtering and bucking but in trying to restart numerous times the ECU fuse blew and left me stranded on a 4-Lane.

IndyLexus

Well, it appears it was the fuel pump. I replaced the Engine Coolant Temperature sensor and that made no difference. Had the ECU checked and they said it was ok. So on 8-24-2012 I replaced the fuel pump and it has run perfectly since then. We have only had a few 80 degree or above days since then. But so far no problems at all. I have only driven it up to about 40 minutes at a time, so no really long trips yet. But before the fuel pump replacement that would not have been possible without the car running rough and eventually stopping.

I also deliberately ran the gas down to less than 1/4 of a tank to see if that would cause the sputtering. But again it ran perfectly. So I have to say it was definitely the fuel pump.

The only other thing I fixed was to remove a huge, and I mean huge, mouse nest from on top of the engine. It was under the air intake between the valve covers. It was mostly made of fiberglass insulation. Really invisible until I removed a lot of parts to replace the ECT sensor. Pulled out a pile of fiberglass the size of a volleyball and a few fried mice. Thought maybe it could have affected the temperature of the engine but I doubt it. I don't drive the car for months at a time in winter so mice can be a problem, even in the garage.

So again it appears the problem with my car was the fuel pump. I replaced it myself, $50 for the pump. I still don't know why it only ran badly during hot weather. But what a relief to finally solve this problem.

Galex

I have been reading these threads for some time, as I have had the same problem with
my 91 LS400 quitting after running for a couple of hours in a hot environment. I replaced the fuel filter with no improvement, then the fuel resistor with uncertain results. The last thing I did was replace the fuel pump, which seemed to go OK. However, after I had put everything back together and then went to fuel the car, I had a new problem. The auto shutoff on the gas pump would stop the fueling within seconds of starting to pump gas. I had to nearly meter the gas in by drops to get fuel into the tank. What would cause this? I need help as I am not a mechanic and this is a huge
inconvenience. Thanks for any advice.

Guy

ViP303

Pretty sure this problem will pop up again down the road. I'm pretty sure its the IACV (Idle Air Control Valve). My 93 LS400 does this. Its not getting enough air to run so its basically suffocating and trying to get air. Your car will idle very low when the engine warms up and when the car is stopped, it might stall or want to die. I still need to change mine but pretty sure its that problem. I put it in neutral and it doesn't want to die until its in drive.

LScowboyLS

it was the problem with mine, had to replace the bearings, but they were not available from Japan for months, and wasn't gonna spend $1000 to have a new one put in, so in the meantime, I had to invent the poor man's Idle Air Control Valve

ViP303

Hell yea LScowboy. I just looked up the price of 1 and the cheapest one is around $500. Im not spending that kinda money on that piece. But I also dont trust used parts. Might just have to take a risk and buy used.

LScowboyLS

there are only two things that can be wrong with an IACV, coils and bearings, and it's usually bearings, which can be replaced - but the bearings are hard to find, might be a special order, so use my poor man's Idle Air Control Valve until you can get a used one from ebay for $50 and the two bearings to rebuild it like new for another $50

Dtwillis5

Have you check the o2 sensors ?