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1991 Ls400 Surge ?


Danny Martin

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Hi first time post but been dreading your post trying to figure out why my 91 LS is surging between 800 to 1100 RPM's ? I cleansed the trottle body and trottle sensor , checked for broken vacume lines . I put sea foam in the tank & new air filter . Gettin ready to replace the fuel filter today . It's a 91 with 200,000 miles on it . Also where is the vacume canister located at? Thanks Danny

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Hi Danny and welcome to the club! A couple of additional questions for you which might help the members determine the root cause. Any codes showing? Does the engine run better when cold or hot or does it make any difference? Does it run ok other than the idle?

Here is how to check for any codes(you should try this even if there is no check engine light) -> http://www.lexls.com...odereading.html

For the canister, see this thread -> http://us.lexusowner...e__hl__canister

My initial thoughts were Idle Air Control Valve or the ignition coils. The coils are infamous for getting weak with age. The IACV for gumming up. Looking forward to more info.

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The car runs fine for about 5 minutes till it warm up.

Hmmm...it sounds like a "closed-loop" issue. With codes 24(intake temp sensor) and 25(lean mixture), I would check the MAF sensor and temp sensor in the throttle body.

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Someone said you can't clean the MAF on a 91 ls400 if so I can't afford a new one $1200.00 heck that's more then the car is worth .

[edit] The air is a "hot" wire type where the air changes the resistance of the wire. There is also a thermal sensor to feed temperature info to the computer. Sometimes the little circuit board soldering gets messed up and disturbed by the cleaning. The MAF is very expensive from the dealer so the best bet is to buy a used one from a junkyard. I got one for my ES300 last year for $50 at the yard. Your symptoms and codes seem to imply a sensor is acting up and, at the moment, the MAF air flow and temp sensing is most suspect. If you are handy, maybe you can carefully pull the unit apart and check the condition of the connections and solder? I think there is a shop procedure to checking it. Will do a search later on.

By the way, Danny, did you know that you can edit your existing post if you like?

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Took the MAT off and didn't see anyway to clean it it was enclosed in plastic . Reinstall and started the car unplugged the MAF and it didn't change anything still surging . So I took for 20 mile drive same thing but now the check engine light is not on . Does this mean the MAT is shot ?

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The LS400 uses a "hot" wire air flow sense and not the "vane" type in the ES300, so I have corrected my previous post. It is very possible that your MAF sensors are shot but lets see if we can find an excerpt from a shop manual on how to check it. Stay tuned...

Ok, I found this but it may be beyond typical equipment -> http://easyautodiagnostics.com/111/MAF-LOOK-UP/MAF-Lexus-lookup/Year-LS400.html

Another question: was the engine running fine BEFORE you cleaned the MAF?

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  • 3 weeks later...

OK I replaced the MAF sensor no change still surging . so now can I clean the IAC valve ? So far I cleaned the throttle valve change the air filer and fuel filter oh I also cleaned the throttle sensor and nothing has helped .

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I quite and gonna scrap it. I cleaned the air control valve today and change a thing . Thank you all for your help. Danny in Arkansas I on fixed income and can't keep throwing money into it .

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check the tubes connecting the maf to the throttle body. make sure they're tight. look for cracks as well. is the IAC receiving a vacume signal? check connections on plug wires and coils also. hate to see this go to scrap.

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