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My Chrysler LHS 1997 3.5L motor idling too fast since throttle body screws were adjusted

Hello and NEED HELP in OHIO
In several years no one has figured this car out , it is a 1997 Chrysler LHS with 3.5 L motor that idles too HIGH when it starts above 2 rpms and higher , it races , sounds like it might blow up idling too FAST . Years ago with a carberator motor you could rev the motor by stepping on the pedal to change carberator setting and hope it to idle down soon…well this car is not going to idle down with out something fixed. …can you help?
When I first bought this car second hand it seemed to idle - ( too slow when in gear at traffic light ) . So, my father and I decided we would idle IT UP…little did we know NOT to mess with thoses screws when sealed from the factory and now we know why. My Father passed away since so have been trying to get help with car for years since. No dealer has figured it out, just charges me a lot of money for incorrect answers. We took the metal cap off of screws on throttle bodies ( dual on that car) and turned them up and counted the turns so we could put it back if needed too. Well, when we idled it up it idled ok until we shut the car off . Then we went to restart the car it idled ( TOO HIGH ALL the time). We unhooked the battery and tried everything to rest it etc. and then We tried to put screws back down to original position but it STILL idles TOO HIGH and races when it is started and never idles down. So nothing we did ever changed it. When car starts it starts above 2 and half rpms and sometimes almost 3 rpms. If I put a load on motor like A/C on fan etc. it helps idle it down a little. Goes down the road at 35 mph with foot off accelerator and hard on brakes.

Question is since I have tried everything by replacing it on car and it did not help and am out of ideas and money can you help? I replaced TPS sensor and every other sensor by throttle body and even replaced the fuel pressure reulator and intake manifold because dealer told me it must leaking air etc. and you can smeel too much gas ( but never see it) . When you open hood you can always smell gas if you are starting the car BUT YOU NEVER see any gas leaking anywhere… I am to the point of thinking it has to be one of the following things…

  1. replace the dual throttle bodies because we messed with the original settings ( and I have cleaned them too ( the inner flaps) thinking they might have carbon build up keeping them from closing enough and even checked the accelerator out to see if it was binding up for it feels like the pedal needs to come back farther) but that is not it either. So thinking we need a working junk yard new throttle body …or …
  2. replace BOTH the throttle bodies ( dual) and the PLENEM because we messed with the original screw settings …from a junk yard that was working correctly. To eliminate wonderig if the whole thing is messed up now…? or…
  3. Replace the computer… ( maybe we messed it up by changing the screw setting to where they can never read correctly)?? Something sure has been a problem all these years because we changed the original screw settings even though we put the screws back in same place later. We disconnected the battery several times for even several hours thinking we needed to reset the computer that way???

Thank you ,

Have checked for air leaking like hoses etc. have had it on computers etc. NO HELP. Problem does not show up on computer read out. Also have replaced many sensors etc. Off and on have worked on this car for 4 years and no one has been able to fix it including dealers. Can someone else that has had this trouble help? thank you Sue

The first thing to check for is a defective Idle Air Control valve. As the name implies, this valve allows air into the engine anytime the throttle is released and this allows the engine to idle. These valves can malfunction where they go to the wrong position for the idle condition and the idle speed is too high. One way to check for a defective IAC valve is, while the engine is idling high, take the handle of a screwdriver and rap on the IAC valve. If doing this effects the engine idle speed the IAC valve is defective.

If the IAC valve checks out okay, then there may be a problem with a defective coolant temperature sensor for the computer. If the defective sensor is telling the computer that the coolant temperature never gets above -20 degrees, the engine will idle very high all the time.


Questions: Is the check engine light lit? You say the accelerator feels like it never returns all the way to where it should be. (paraphrasing) Have you checked to make sure the throttle is closing all the way? Also it could be something weird like the cruise control actuator. Make sure that isn’t opening the throttle partway. Finally, it could be something like an intake manifold gasket leak where it’s sucking air like the throttle is open all the time.

have checked the IAC and replaced it a long time ago…did not fix it but thank you for input. I never heard of the defective collant temp sensor causing that probem but could be? there are a few different temp sensors which one is for the computer?? will check into that for a possible fix.? I ALREADY replaced the manifold gasket a long time go too, did not fix it. I also have checked out the throttle body and cleaned it many times making sure the throttle was closing but thank you for the input. My cruise control works and I tried unplugging it before so as to eliminate that not causing any problem and it did not fix it. Is there something else I should of tried with the Cruise to make sure it was not causing the trouble?? I will keep trying to figure this out and appreciate any help out there. thank you Sue

The screws you messed with are the throttle stop screws. You should back them off until you see a gap at the end, then screw them in until they just touch. Their function is to keep the throttle plates from damaging the throttle body.

thanks Keith, have done that too. What I am wondering most is this? Since NONE of the things I have done to correct this high idle has worked am wondering the NEXT STEP to take? Either REPLACE the computer since nothing else has worked? Am wondering if it just can not be RESEST?? so would replacing it TAKE CARE OF THE HIGH IDLE AND SET IT BACK TO ORIGNIAL POSITION
OR…replace the throttle body?? since I messed with the original settings and maybe it is just too far out of wack now and can not be corrected even though I moved the screws back. (Out of these 2 things ) which do you chrysler mechanics think is the trouble?? I have done everything else and nothing has worked so which of these two things I just mentioned would be the most logical to fix it. ?? thanks in advance for any help! I am at the end of ideas on this and end of money!

try spraying ether around intake manifold and see if motor revs up a little. sounds like it is sucking air in from somewhere and computer is adding gas to compensate for the air.

I have had this same problem and spent alot of money trying to fix it… the problem is your gasket that goes between the throttle body and intake manifold the part cost two bucks and takes 15 minutes to change there’s only four bolts to unscrew and two sensors that u have to unplug pull it back take off the old gasket that is worn and put the new one on make sure to tighten pretty good and don’t forget to plug in your sensors…hope this helps. Youo

Any chance you could tell us what the Throttle Position Sensor Output voltage is?? One it would tell us if the throttles are still being held open by the adjusted throttle stop or may indicate an issue with the TPS/Circuits. The module has a minimum threshold voltage for idle detection… If the TPS voltage is above this threshold, the pcm assumes that the vehicle is in part throttle mode. The PCM at that point will open the AIS (Automatic Idle Speed Motor) and consequently increase the engine rpm.The TPS is a three wire sensor (one is v-ref/5.0 volts, one is ground zero volts, and the third is sense/variable). IF you can back probe… we would need to know what the sense voltage is. Besides, upper and lower manifold air leaks, I have seen TPS installation issues as well as circuit issue causing tps voltage high events (This will not throw a DTC) .

Another question would be whether you had the throttle bodies off??? Did you disconnect the crossshaft that connects the two bodies? Did anyone attempt to back the throttle stops off at least a little?

How is the condition of the fuel rail gaskets that seal the injector manifolds to the intake/heads? This could be easily verified if you had access to a scan tool. We would need to know a couple of module parameters. TPS voltage, AIS motor counts, Fuel trims short and long for both banks, coolant temp and IAT temp, MAP sensor would be very beneficial… If you had a vac leak, you should see a valid TPS signal, but an AIS count being very low and likely fuel trims pushing significant positive numbers/fuel compensation.

If the AIS counts are high, look at the tps voltage… (Its been a long time since i worked on these at Chrysler dealer) .95 volts was nominal closed throttle value (Again if memory serves me correctly) once you get over 1.05-1.10 volts, the module will start to assume part throttle mode. There is also a learned minimum TPS value that is also critical, but being that it appears that this condition occurs all of the time, I will leave that discussion out for now.

All in all this should be a very easy thing to fix or at least diagnose… Let us know what you find or what tools/equipment you have access too.

thank you for the help. Sorry I do not know more about technical terms nor do I have any devices to check parts. But, one thing I forgot to mention is this. When I first got the car I felt it idled TOO SLOW when I was sitting at a light. So, I thought it needed idled up? That is when we changed the the 2 screw positions under the caps. The 2 screws were the outer ones. There are 4 there on the dual throttle body but we only chaged two to idle it up. Seemed to work until we turned car off…then we went to start it again hence came the high idle. So, every since then it has the high idle no matter what I seem to try. I feel we over compensated for the SLOWER idle probelm it had in the beginning by changing the screws up but that was probably not the real problem? I will try to look over the advice the last two gentlemen gave me and see what I can come up with. Today am tearing the plenum back off and several other things and changing the throttle body hoping that will help. thank you again, Sue

UPDATE: Since I last posted on here I have performed the following on this car.
I have been told by many mechanics ( locally I mean that were persistant in telling me they were right and were not) to go ahead and replace the air intake gasket and I had already done the plenum gasket 2 years ago because a mechanic told me to do that. Last weekend I redid the plenum and went farther down and did ALL the gaskets etc. AND put another dual throttle body on it to eliminate the POSSIBE throttle body set screw problem we did years ago. I have done all these things at the opinion of professional mechanics that were not lstiening to me when I said all that was wrong with the car in the very beginning. You know the gtype…know it alls that do not listen too well. So I am not angry at anyone but if people would just listen and get all the facts a better diagnoses can be made so that the customer does not have to spend so much needless money on things over the years that correct diagnoses would tell you you did not need that part or parts. All these years INCLDUING chrysler garage has diagnosed this car WRONG!!! I just can not give up on it so I keep trying.
So now it has all the new gaskets O rings and factory set throttle body and made NO difference! I went ahead and put in new a new fuel regulalator too 9 whcih was expensive for someone told me I needed that ( local mechanic) ) and later saw I did not need that it , it was only the O rings in fuel rail and injectors that were bad. And the O rings etc for fuel rail and fuel injectors ( which by the way is rediculous to try to find) I found the O rings in a set for the fuel injectors BUT NO ONE including Chrysler dealer has NO fuel rail or return line fuel O rings which are very important! Finally had to find the closest O ring type in an Air conditioner O ring and used them. * ( They were not the right size but no one carries anything else for those type of O rings). Redicuous running around and nothing out there for those type of O rings. Anyway, all that I did to changing gaskets did NOTHING for the high idle problem as all these so called expert mechanics told me i needed to do. Much money later and agrivation I still have the same problem. So, this is my own annalysis. 5 years ago I said there had to be a problem already with something because it idled too low when it was in gear so I was afraid it would start stalling on me so that is why we tried to move the preset screws on the throttle body to idle it up. Well it did idle up and NEVER idled down again… so my thoughts are this. The EGR valve is behind the motor ( hard to get at) but I suspected it being bad years ago and everyone said NO, it was not that. I conclude that it IS that and ties directly into the ECM which holds the idle speed memory as well. When you spray carb cleaner to back corner of motor it lowers the idle speed and it has been that way for hyears. Everyone said oh put the air intake and plenum gaskets in but it was NOT that. I say the EGR is bad in meaning has a stuck diaphram or leak ( not in the lines going to it though) and something is wrong with it and makes it idle too high since we messed with those original screws and the ECM some how over compensates for the low idle it once had with the high idel from wrong signals or problem in EGR to ECM. I do not know the exact language in technical terms but to me eveything points to the EGR when everyone has told me (in the past that it was NOT the EGR valve and components.? I said then I was suspicious of the EGR WAY BACK IN THE BEGINNING that somehow it is the problem that still exists now. I will let you all know when I can afford to get one and figure out how to get it on. It just can not be anything else to me and am at my last attempt to fix this car…LOL
I know it idled TOO low when I first noticed the problem and then after we over compensated that with changing the idle up and it went TOO high and would never come back down it must be that and confises the ECM with its wrong siganls??? Everythiing to me points to the EGR…BUT, it is expensive and hard to get at so anyone have any knowledge on it? The car has the 3.5 motor. When I get the money saved to buy the expensive EGR and accompanying selinoid that goes with it and get it in I will post to tell you if I am right after all these years! LOL This is driving me crazy and very expensive listening to others that TOLD ME they KNEW what was wrong with the car and in fact DID NOT know. lol I have replaced everything else on the car inclduing Temp control switch TCM towice as a matter of fact and replaced the IAC twice and all the other things mentioned above…wish me LUCK! thank you all for your help and comments. Sue city Sue

I’m a bit blind from reading all of this and don’t mean to come across as rude but I would ask the following question.
How much time and money have you spent on mechanics at a shop to sort this problem out?

  1. The idle speed control motor has an air supply tube connecting it to the plastic air cleaner plenum. Disconnect the 1/2 inch tube from the idle speed control motor, with the engine running do you hear air rushing through the opening? Block the opening with you thumb, the engine should slow and nearly stall. If there is no change; step 2

  2. Pinch the vacuum hose to the brake booster with a pliers to block the booster if it is leaking.

  3. Disconnect the EGR tubes from the intake plenum, don’t disconnect the tube from the EGR valve.
    Next make a blocker plate from cardboard (paper board) and install in place of the EGR tube gaskets. This will eliminate the EGR system for testing purposes.

Let me know if this identifies a problem.

“When you spray carb cleaner to back corner of motor it lowers the idle speed and it has been that way for hyears.”

Whatever you are spraying the carb cleaner on is where the problem is. You need to pinpoint the exact spot that has the greatest affect.

The intake ports near cylinders #5 and #6 are a common place for intake manifold gasket failure. I trust the intake manifold and plenum gaskets were just replaced? A cracked or loose EGR tube can cause this, its difficult to see. I can’t believe nobody can find this big leak.

Never should have adjusted the set screws
If you can borrow an actron eng analyzer you can set the engine rpm with it if everything else is ok. also the engine idle air control motor may need to be replaced.
Do the OBD test (ON BOARD DIAGNOSTIC ). Using the key do the following test turn the key on -off /on-off /-on (do not turn on the engine).count the engine testlight blinks it should blink (blink-- blinkblink) = 12 start of test then it will start blinking the fault codes 2 numeral codes. Your car may readout on the trip odometer my 95 reads out blinks of the check engine light. You need to get a haynes manual. I replaced the idle air control motor and set the rpms at 800 rpm . Now all ok. You will need a manual or go to a chrysler and asl what the code is for your rngine is. On my 95 3.5 L ,the code is 25 for EIAControl
EGR solenoid circuit may be defective. code 32 it controls an egr valve valve and solenoid are a matched set.
Hard to get at and cost is $125 parts
THis May not be effective if the vanes are not properly set as airfloe into both intale manifold vanes are set properly and airflo is balanced. If only the codes 12 and 55 are displayed no codes have been set. 55 code is end of test…
You will have to use a diagnostic code reader for complete test.
Good Luck

just repaired a 1999 L H S 3.5 that was running at a high idle when you start it up. I worked behind some one else; I did replace the upper and lower intake gasket.The problem was bushings! What i asked the dealer for; Were orings they look like bushings to me they were ten dollars each they go in with the flange part up in the rear of the upper intake under it. Their are two emission tubes that are mounted on the rear of the motor held to the upper intake by to clips they pull right out rpms are now at 750

Thanks for the feedback.

Oops, I just realized that you are not the OP (original poster) and that you hijacked this thread. The information you provided is useful so thank you for that, but I mistook it for feedback. That is a shortcoming of this site, the OP is not listed in the title