92 Ls400 Rough Idle, No Acceleration Fine At 12V Fuel Pump Relay Other

[JTom]

Hey guys friend of mine is working on my LS400, I have really rough idle which seems to be a common issue here and stalling while driving has happened twice, no acceleration at idle, and problems are getting worse over time. However with the fuel pump relay at 12V things are working fine, changed the fuel pump relay resistor based on another tip here of someone with the same issues, that didn't solve the problem. Any other ideas? Don't want to do the fix and replace, could it still be the fuel pump even though it works at 12V?

[steve2006]

You 1st need to determine whether the problem is fuel or ignition related.

If you have the required equipment you can measure the fuel rail pressure to eliminate a problem there.

A common cause of the problem you describe is the failure of the drivers side ignition coil.

Also obtain the fault codes if any are present to assist in possible causes.

[curiousB]

You 1st need to determine whether the problem is fuel or ignition related.

If you have the required equipment you can measure the fuel rail pressure to eliminate a problem there.

A common cause of the problem you describe is the failure of the drivers side ignition coil.

Also obtain the fault codes if any are present to assist in possible causes.

ditto the above. You have to be sure if its idle issue only or at all speeds. If all speeds then it is more likely ignition as fuel starvation would be more of an issue at higher engine loads. If it’s a bigger problem at higher engine load it could be fuel pump, pressure regulator, or (clogged) fuel filter.

There are two fuel pump relays one to turn it on the other to switch in a series resistor which gives a two speed operation to the pump. If it gets stuck on high speed (resistor shorted) you shouldn’t see any problems but could wear out pump faster than with normal two speed operation.

Most users seem to have ignition problems on these cars more commonly than fuel problems. I’d rule out ignition first.

[lex460]

You 1st need to determine whether the problem is fuel or ignition related.

If you have the required equipment you can measure the fuel rail pressure to eliminate a problem there.

A common cause of the problem you describe is the failure of the drivers side ignition coil.

Also obtain the fault codes if any are present to assist in possible causes.

ditto the above. You have to be sure if its idle issue only or at all speeds. If all speeds then it is more likely ignition as fuel starvation would be more of an issue at higher engine loads. If it’s a bigger problem at higher engine load it could be fuel pump, pressure regulator, or (clogged) fuel filter.

There are two fuel pump relays one to turn it on the other to switch in a series resistor which gives a two speed operation to the pump. If it gets stuck on high speed (resistor shorted) you shouldn’t see any problems but could wear out pump faster than with normal two speed operation.

Most users seem to have ignition problems on these cars more commonly than fuel problems. I’d rule out ignition first.

The usual way to rule out fuel flow and pressure problems is to hook up the fuel pressure gauge tape it to the front window and go for a drive.

[fsuguy]

lex460,

Can you post some pics of how to hook up a pressure gauge to the fuel rail? I have a '92 and want to check my fuel pressure 'cause once the engine has warmed up, I lose performance drastically! Any help in this area would be very much appreciated.

TIA

Mike

[lex460]

lex460,

Can you post some pics of how to hook up a pressure gauge to the fuel rail? I have a '92 and want to check my fuel pressure 'cause once the engine has warmed up, I lose performance drastically! Any help in this area would be very much appreciated.

TIA

Mike

Hi Mike I found a hookup for the 430 and specs for the 400 and SST info hope this helps.

pg 11-8

11 - Fuel System.pdf

sst.pdf

serdat.pdf

[Quack11]

I had this same problem with my 91 LS 400 and the fuel pump was the problem. Everything read and checked fine witht he fuel system but I found out the the pump was not delivering enough fuel when the engine was under load. Good luck.

Hey guys friend of mine is working on my LS400, I have really rough idle which seems to be a common issue here and stalling while driving has happened twice, no acceleration at idle, and problems are getting worse over time. However with the fuel pump relay at 12V things are working fine, changed the fuel pump relay resistor based on another tip here of someone with the same issues, that didn't solve the problem. Any other ideas? Don't want to do the fix and replace, could it still be the fuel pump even though it works at 12V?

[fsuguy]

lex460,

it's been a while since you posted the information, so please accept my apologies. Thank you for posting this information. Will post what I find after my efforts.

Mike