1990 Grand Prix


Old 06-28-06, 06:00 PM
Thread Starter
Join Date: Jun 2006
Posts: 1
Angry 1990 Grand Prix

I have a 1990 Grand Prix SE. When I come to a stop or a red light, the car runs rough, but only when its warm. It lopes and the check gages light comes and stays on. Spent $200.00 last week - repairman put all new sparkplugs and wires on, had it for 3 days and it doesn't run any better than when I took it to the guy.

Took it to a dealership the other day and waited around for 3 hours while they diagnosed (more like guessed) what the problems could be. According to them the car is registering that I driving high up in the mountains in a different altitude and my vacuum modulator is malfunctionning - sucking Trans fluid into intake/also map sensor reading incorrectly-recommends R&R map sensor. O, Vacuum Modulator BG injection/induction service/found EGR code in history-recommended R&R EGR valve/Tech noted that Oxygen sensor could be damaged from trans fluid/Rec R&R sensor/notes that started is starting to go bad. OH....but no guarantees and could run from $130.00-$1050.00.

Can anybody offer any advice, input, I don't want to have to put this kind of money into a car that is only worth $800.00 blue book. Seems neither the 1st nor the 2nd people know what in the world they're doing. Please Help! Thanks
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Old 06-28-06, 06:37 PM
Join Date: Jun 2006
Posts: 338
Clean the throttle body and idle air controller. Put a hose over the throttle body that is connected to a compressor and pressurize the intake to about 7psi fix all vacuum leaks, get carb/throttle body cleaner get rpms up and spray it in for 3sec and wait 5 then repeat till can is gone. Put fuel system cleaner in tank.

With out being able to monitor anything first hand their isn't much more I can tell you.

Bad MAP reading showing that you are in high environment means the map is bad. I wonder if the mechanics got it right for that reading if you have a vacuum leak it will read what it does when the key is just on engine not running or near it.

Not sure what your saying here "vacuum modulator is malfunctionning - sucking Trans fluid into intake/also map sensor reading incorrectly-recommends R&R map sensor." I think the translation from the what the shop said got messed up here. I am not familiar with transfluid flowing through or near intake.

First thing I would do is go to a local parts store and get a map sensor. That is crucial to a good running motor.

A bad egr can also cause a crappy idle.

Map sensors are easier to change than changing oil. Buy the part and hunt around usually on the top of the motor somewhere and replace it, shouldn't be more than one or 2 screws and a tiny hose.

EGR valve is the same way easy as cake.

The oxygen sensor you may need to buy an oxygen sensor socket. That is located on the exhaust with a wire hangin out the top easy as cake to change to.

Last edited by hotrodder89; 06-28-06 at 06:50 PM.
Old 06-29-06, 04:20 AM
Join Date: Dec 2001
Posts: 2,538
check guages light is generally associated with the guages on the dash if one is reading inccorectly or out of range it will usually come on do you notice any guages that are not reading normaly when this light comes on.
how about a check engine light does it ever come on?
I would not suggest replacing the sensors or egr valve at this point however if your vacum modulator is leaking transmission fluid would probably suggest you fix it first if there is any transmission fluid in the vacum line going to the vacum modulator the modulator is bad and needs replaced.
cleaning around the throttle plate with carb cleaner by removing the air intake hose and either clean it while the engine is running and varying the throttle, rpm so it doesnt kill the engine spray carb cleaner into the intake, or you can do it with the engine off and hold the throttle open and clean around the throttle plate but it will be a little flooded when you go to restart the engine if you do it with the engine off.
then I would suggest disconnecting the battery cable for about 30 seconds to clear any codes that may be stored in the computer and drive the vehicle for awhile before retesting for codes to see if any came back while it will probably need a sensor or even more work I doubt it would need a map sensor or egr valve to fix the rough idle problem but it is an older car and may cost more to get everything fixed than what you want to put into it.
Old 06-29-06, 08:33 PM
billys68ss's Avatar
Join Date: May 2003
Location: USA
Posts: 1,459
The first thing to do is fix what is broke. Like the modulator valve. If in fact you are getting trans fluid in your intake via the mods vacuum line then the valve is bad and needs replaced. The fluid getting into the intake will cause all of the other problems. It is diluting the fuel charge and will be very noticable at idle. It is almost like trying to run your gasoline engine on diesel fuel. If you decide to clean your throttle body then I highly recommend removing the throttle body from the intake and removing the idle air control motor. Then cleaning these items separately. The problem with just spraying carb cleaner down the T/B is that you are not really doing anything but cleaning the throttle blade. The IAC ( idle air control motor) should be cleaned separetely because no amount of carb spray down into the intake will do this. The seat where the IAC is mounted gets pretty nasty also and should be cleaned. Do not use a pick or anything hard to scrape the carbon build up off. You will damage the parts and make matters worse. I use an acid brush and keep it wet with carb cleaner. (thats what it is called, not what it does) Its just a steel brush with camel hair like bristles.
Hope this is helpful to ya,
Old 06-30-06, 11:16 AM
Join Date: Jun 2006
Posts: 338
You can leave the throttle body on when cleaning you just have to hold the flap open and stick your fingers in there with a rag to clean the sides and back of the plate. You don't need to take the throttle body off to clean the IAC there should be a couple of screws and it comes off.

If you don't want to use carb cleaner cuz it says carbs you can buy throttle and intake cleaner because it says throttle on the bottle.

Last edited by hotrodder89; 06-30-06 at 12:01 PM.
Old 09-15-06, 10:30 PM
Join Date: Apr 2004
Location: Colona
Posts: 6
90 Gp Se

I also have a 90 GP SE it just went over 150000 & still runs great. It has a constant check gauge light but all gauges are reading normal, the only fix is to replace the instrument cluster which I did back in 1993, I wont do it again.
If you do remove remove the throdle body, replace the 2 bypass hoses that connect to it. I had 1 blow on the highway in the winter it wasnt fun.

The question I have is my EGR valve has never been replaced & at 180-200 bucks I wont just do it for nothing but what I want to do is pull it & clean all the carbon off it & the intake, will this hurt anything? or would I be better off doing nothing.
Old 09-15-06, 10:35 PM
billys68ss's Avatar
Join Date: May 2003
Location: USA
Posts: 1,459
It will not hurt anything to do this. It may actually keep the valve from failing as they tend to get gunked up on the pintle. I assume from the price range you quoted that this is the electronic EGR valve. I have cleaned some and I have replaced some over the years. Replacing them has not always been the fix, but neither has cleaning them. But more often than not it prolonged the agony.
Good Luck,

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