1991 Ls400 Hesitation At 65+ Mph

[abbakins1983]

My 1991 LS400 is at 210263 (to date 12/16/11). When I bought her, I knew it was an investment that would take time and money...mainly money since she is pushing legal drinking age.

I have had a hesitation when trying to go at or over 65+ on the highway. I have replaced the plugs/wireset, checked air filter, new fuel pump/filter, O2, exhaust...everything including the small items. I have even replaced the water pump, obviously along with the timing belt. It feels almost like the fuel is being cut. If I slow down, it will stop...

So, my question is:

Is there a governor or speed limiter on my model? If so, (which I haven't been able to confirm yet) I understand it would be a tangible item instead of a code since it was made before 1996? I do not have a main mechanic. I either DIY or take it to whomever gives me facts, are straight to the point, and fair on rates. This being said, no one will answer my question over the phone. They want a $100 diag before they could tell me. I do not want to shell out for a diag if it is simply a bolt.

Also, what's up with the speedom? The tac works fine, as well as the fuel gauge, no issues with temp gauge. But the silly speedom just lays all the way down and will be at zero mph while going about 60 mph. Need to be tightened?

Thanks in advance,

abba

[curiousB]

Its your conscience telling you to slow down… :chairshot:

But more to the point it is either fuel or spark. I am temporarily ruling out tranny because it has a lock up torque converter so shouldnt be subject to slippage.

If it were fuel I would expect youd see the hesitation at lower speeds, say 50MPH, but with aggressive acceleration (i.e. heavy engine load, thus more fuel demand). Maybe try that next time on a highway. Does it bog down at 50MPH with heavy acceleration?

Another test might be to note the RPMs at the time of the hesistation. Then down shift one gear and drive at a speed that you get about the same RPMs and see if engine hesitates there. That might indicate ignition at any speed where RPMs go over __X__. These cars are known for problematic ignition coils so you could try replacing one and see if it fixes it. If not swap the good new coil to the other side and see (Its unlikely two coils have simultaneously gone bad) if the problem goes away.

A last possibility is maybe a plugged catalytic converter but I think that is less likely and youd probably see other symptoms.

[abbakins1983]

LOL! I actually realized it did this when I traveled for the first time out of town in her (Purcell, OK). I tried keeping up with the flow of traffic...not so much.

I will take a better look at her in the daylight tomorrow. I am going back to OK tomorrow afternoon and can better test her out without traffic and such. :whistles:

Thank you so much for the pointers and when/if I figure it out, I will post it.

[1990LS400]

Yes, the top speed of your LS400 is electronically limited - to just under 150 mph. On European LS400's of all model years, the top speed is electronically limited to approximately 156 mph.

[jaswood]

My 1991 LS400 is at 210263 (to date 12/16/11). When I bought her, I knew it was an investment that would take time and money...mainly money since she is pushing legal drinking age.

I have had a hesitation when trying to go at or over 65+ on the highway. I have replaced the plugs/wireset, checked air filter, new fuel pump/filter, O2, exhaust...everything including the small items. I have even replaced the water pump, obviously along with the timing belt. It feels almost like the fuel is being cut. If I slow down, it will stop...

So, my question is:

Is there a governor or speed limiter on my model? If so, (which I haven't been able to confirm yet) I understand it would be a tangible item instead of a code since it was made before 1996? I do not have a main mechanic. I either DIY or take it to whomever gives me facts, are straight to the point, and fair on rates. This being said, no one will answer my question over the phone. They want a $100 diag before they could tell me. I do not want to shell out for a diag if it is simply a bolt.

Also, what's up with the speedom? The tac works fine, as well as the fuel gauge, no issues with temp gauge. But the silly speedom just lays all the way down and will be at zero mph while going about 60 mph. Need to be tightened?

Thanks in advance,

abba

How is the level at WOT (Wide Open Throttle) acceleration up to 65MPH, say at 45MPH...? Do you consider it to be adequate..???

If so it very well could be, given the lack of an indication otherwise, that the engine/transmission controlling ECU "thinks" it's at the electronically enforced speed limit.

[steve2006]

I read the post 3 times then the last section with the speedo issue made me think this could be down to a speed sensor issue.If there is no speed sensor signal the speedo will not work as it doesn't but this signal also has feeds to various ECUs including the transmission and engine and could be the cause of your problem approaching 60 mph making the engine go into limp home mode to protect it.

I think the sensor is mounted on the passenger side of the transmission and it may be a good idea to check the sensor and wiring.