Question about 1991 Pontiac Grand Prix

8 Answers

1991 Pontiac Grand Prix 3.4 liter rough idle and acceleration

I have a 1991 Pontiac Grand Prix 3.4 liter that is running extremely rough/erratic at idle (especially when the engine is cold). When I'm driving and press the gas pedal to the floor to pass, the vehicle stalls momentarily and then takes off.

When scanned, I get codes 33 (MAP/VAC Sensor High) and 43 (EST Failure).

I've replaced the MAP sensor and it seems fine. I believe the 43 code is my problem and is probably the Knock sensor. When I conduct a datastream, I show no knock sensor signal and the ignition timing adjusts as far as 28 degrees at times.

Do you agree that the knock sensor is probably the best place to start? More importantly, where the hell is it at on a 3.4 liter in 1991?

Posted by on

  • 10 more comments 
  • Chris Hall
    Chris Hall Mar 06, 2009

    Where is the knock sensor located on a 3.4 liter in 1991?

  • Chris Hall
    Chris Hall Mar 06, 2009

    crisyszone, am I correct that there is only one knock sensor on the 1991-1997 3.4 liter? Whats the best way to access it? I think I see what you're talking about, but it looks impossible to reach....

  • Chris Hall
    Chris Hall Mar 06, 2009

    Whats the best way to access it? I think I see what you're talking about, but it looks impossible to reach....

  • Chris Hall
    Chris Hall Mar 06, 2009

    Hari, if there are two, can you please give a better description of where to find them? What color are the wires coming from them? What is the best method of accessing them for removal?

  • Chris Hall
    Chris Hall Mar 06, 2009

    emissionswiz, well, the car was taken to a shop where they used the shotgun dagnostic approach and started randomly replacing parts based on nothing but a hunch. During this process, they installed two new proms/ecms. Both times, the vehicle went from running poorly to not starting at all. So, they decided that both NAPA new proms must have been defective and they went and got a used combo from a wrecking yard. When they installed it, the car would start again but had the same problem it had in the beginning. I assumed that the problem with the two new proms was that they were not installing them correctly. But, when the used ecm/prom combo yielded the same problem that the original parts did, I assumed that this must not be the issue. Also, I have a genysis scanner. In datastream mode the knock sensor is reading no signal and 0 degrees while the engine is adjusting the timing to 28 degrees. I am no expert, but this is the info I used to form my opinion. Having said that, if you think its the prom or ignition module (the ignition module was my initial thought prior to seeing the datastream info), I would be happy to start there. What should a test and how?

  • Chris Hall
    Chris Hall Mar 06, 2009

    Hari, I sincerely appreciate your input. But, sending me a link of where I can purchase a repair manual that will be shipped to me in a couple weeks is not going to work for me.

  • Chris Hall
    Chris Hall Mar 06, 2009

    This is a 1991 3.4 liter V6.

  • Chris Hall
    Chris Hall Mar 06, 2009

    Sorry emission, you've helped me a couple times in the past and have always given sound advise. I'm not ruling out the prom/ecm/ignition module if you think that is what it is. Don't you find it currious that two brand new proms were installed and both were deemed defective by the shop? Then, when they put a used one in the car starts running again with the same symptoms it had with the original. Seems odd to me.

    I'm not sure if the prom is updated. My Genisys is listing the prom ID as 9021.

  • Chris Hall
    Chris Hall Mar 06, 2009

    Thanks GM Elec,

    Vacuum appeared to be ok. I did locate a couple couplings that caused me some concern, which I repaired. But, it did not changed the driveability.

    I decided to pull the plentum in an effort to easier access the knock sensor. After doing so, I began removing the "rear" spark plugs. Imagine my surprise when I removed the middle plug (which is a long plug wires that access the plugs that are bolted appx 8 inches down) and it acted as a plunger that siphoned motor oil out of the hole. Yes, the entire spark plug hole was full of motor oil.

    Possibly a bad PCV valve or vac problem? Or are we looking at a ring, piston or sidewall issue?

  • Chris Hall
    Chris Hall Mar 06, 2009

    Disregard the PCV valve being a possible culprit, it doesn't appear to be equiped with one. Possibly a leak in the valve cover gasket? Typically, I would trace the trail of oil to the source. But, with the hole being completely full of oil, it's impossible to do so. Any ideas of what testing I can/should do to track down the issue?

  • Chris Hall
    Chris Hall Mar 06, 2009

    After I get the spark plug issue (plug well full to the brim with oil), if the problem still exists, I'll check the coil pack.

  • Chris Hall
    Chris Hall Mar 11, 2009

    Thanks

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  • 158 Answers

Hello, yes I would start at the knock sensor. It is located in the front side of the engine I believe it is close to the alternator,near the harmonic balancer. If that doesn't work the best bet is that the powertrain control module is bad and needs replacing. Please rate my answer, Thank you.

Posted on Mar 11, 2009

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  • Pontiac Master
  • 19,396 Answers

Since everything here had been already suggested, I think that you can try replacing the air intake temperature sensor, placed on the air intake, as that can also affect the behaviour of the engine when idling at cold condition.
Get a replacement sensor for few dollars at a local part shop and try replacing it, or try disconnecting the sensor and see how it goes.

Posted on Mar 09, 2009

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  • Pontiac Master
  • 4,369 Answers

Almost always a knock sensor is not bad. It is just reacting to the violent shaking going on due to the engine misfire issue. I believe you have a base engine problem, not an electronic engine control problem. I believe you have a coil pack on this engine with wires going from it to the spark plugs.
Try disconnecting them one at a time from eac cylinder to see if it makes a difference. If there is no change when you disconnect one, that is the problem cylinder.
Be careful to shut off the engine each time you disconnect a plug wire or you will get a jolt. Unless you have plastic spark plug boot puller.
Then check for spark at that cylinder. If no spark, replace the coil pack. If spark, check to see if injector is pulsing. Listen with a hose or stethescope.

Report back. Good luck

Posted on Mar 06, 2009

  • gerry bissi Mar 06, 2009

    Remember, the knock sensor just retards the timing in 3 degree increments until the kspark knock goes away. SO, even if it ws bad, it just would not ****** your timing. The engine vibration is mimicking spark knock.

  • gerry bissi Mar 08, 2009

    Ihave been doing tech assist for many years and never found a knock sensor to cause any problem except not retarding the spark whne there was a true spark knock.



    The spark plug hole being full of oil meqans you have an external leak allowing oil t fill up the hole. So, there is possibly an o ring in the tube where the spark plug goes in leaking oil. None of this oil is getting into the combustion chamber so don't worry about that.

    However, the oil can cause that cylinder to misfire. I owuld get as much oil out of the tobe as possible before pulling the spark plug. I'm sure you will find it is black from not firing. Replace that spark plug wire and see if it is ok now. You could also stick a spark plug in that plug wire and ground the threaded area against the block to be sure that plug is firing.

    PCM's also almost 100% of the time get relaced for no reason. We test thousands of them/year before remanufacturing and almost none test bad. hey just don't fail, ecept very early in life from a manufacturing defect, not a durability issue.



    I would also check the coil pack to ake sure it is ok and the connector going to it is not corroded. I worked on one car and just happended to catch one wire so corroded that it brkoe off when i wiggled it. That one wire ran half the coil pack so 2 cylinders were not firing. I still think all your issues are base engine meaing not the electronci engine controls. Good luck.

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  • Pontiac Master
  • 1,000 Answers

3mac,
If your still in the mood to try and diagnose this issue, there are some other options.
The map sensor is vacuum sensitive, if there is any type of a vacuum leak the map may drop below specified range and set the code. The vacuum leak will also create a lean condition that will enhance detonation and throw your knock sensor into a "tissy".
If the engine is running rough at idle, start disconnecting vacuum accessory hoses one at a time and see if the idle quality changes. Start with the brake booster, disconnect the hose and plug the port at the intake manifold and check idle quality, if no change continue on to the next.
After you have verified NO vacuum leaks, check the EGR valve for pintal contamination. I assume the EGR valve has never been cleaned. You can clean the port of the valve after you remove it with a brass wire brush and pic. Make sure the gasket has not deteriorated and allowed un-metered exhaust into the intake port.
Start there and let me know, we can continue from there if problem has not been found.
Good luck,
Regards,

Posted on Mar 06, 2009

  • Bruce Bracey
    Bruce Bracey Mar 06, 2009

    3mac,

  • Bruce Bracey
    Bruce Bracey Mar 06, 2009

    3mac, While you have the plenum of, remove that EGR valve and clean it.

    Let me know.

    Regards,

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  • Master
  • 916 Answers

What crysis has said is correct and their is only one knock sensor as far as I can tell

Posted on Mar 06, 2009

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  • Expert
  • 289 Answers

Hi..There is two knock sensors on this engine, one is
on the engine block closest to the firewall below the
spark plugs, the other one is on the opposite side of
the engine block closest to the front of the vehicle
beside the oil filter.
check this out..
thank u..

Posted on Mar 06, 2009

  • 2 more comments 
  • Hariharanath V Mar 06, 2009

    hi..check the knock sensor immediately.

    try this out..

    thanks..

  • Hariharanath V Mar 06, 2009

    No,there are two knock sensors.

  • Hariharanath V Mar 06, 2009
  • Hariharanath V Mar 06, 2009

    hi..If this is the 91 Grandprix with the 3.1 eng. the Knock sensor

    is located just under the manifold back next to the fire wall you

    can see it from the bottom. It's hard to get to if you have small

    hands you can touch it from the bottom. However it is much

    easier to let the eng. cool down lean over the eng. close to the

    front of the eng. if you can get your hand and arm back there it

    is much easer to remove and replace from that point.

    try this out.

    thanks..

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  • 154 Answers

Check the senset wired properly.

Posted on Mar 06, 2009

  • crysiszone Mar 06, 2009

    On the side of the engine closest to the firewall. It is almost directly under the middle exhaust port of the exhaust manifold about 2 or 3 inches down.


  • crysiszone Mar 06, 2009

    better you can go using a repair manual.

    for repair manual CLICK HERE.

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  • Pontiac Master
  • 63,801 Answers

Here are the causes of code 43 from the GM fact manual:
Typical causes for this code include:
1) Open or shorted knock sensor
2) Loose knock sensor
3) Excessive mechanical noise within engine
4) Improper or incorrectly installed PROM or CALPACK in the ECM or defective ECM
5) Intermittent open in the EST line to the ignition module
Let's look at them, u say u think the knoc sensor may be the problem, based on 30 years of new car dealer tech work with a specialty of electronic engine control I doubt that that is the problem, do u have the latest PROM? have you replaced the ignition module, which is a more likely suspect? also you asked where the knock sensor is, it is in the water jacket of the block near the bottom of engine near the oil pan seal surface of the block, it is round, if u do replace it make sure u drain the block, also torque the sensor to the correct setting or it will not work properly, but that said I would stick with a PROM replacement and a new EST module.

Posted on Mar 06, 2009

  • yadayada
    yadayada Mar 06, 2009

    Well it would be nice to know all the facts and repairs performed before I try to help u. Good luck. Has the PROM be replaced or updated?

  • yadayada
    yadayada Mar 11, 2009

    u need to fix the oil leak at the plug, the oil will conduct and short the plug out as well as ruin the insulator, then concentrate on the Electronic spark control issue, which is most likely the module or the wiring to the module.

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