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Vehicle cuts out

FANNING88

MEMBER
2006 BUICK RENDEZVOUS
3.5L
6 CYL
2WD
AUTOMATIC
127,000 MILES

I noticed that the idle is off once it warms up some. Also it vibrates the whole car while it is idling or at a stop. Now it surges really bad under load especially at about 65 mph I can see the rpm's jumping around but it don't match up with the gas pedal. I have changed the mass air flow sensor and the throttle body and put new spark plugs and wires in it. The old spark plugs were so bad the electrodes were completely gone. I also changed the ECT sensor the MAP sensor and air filter. I checked the fuel pressure and it is about 60 psi. There is no check engine light on or no codes stored in the computer. I did notice that my fuel trims go negative when I am sitting in park or just stopped in gear.

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Monday, June 10th, 2019 AT 6:51 AM

25 Replies

KASEKENNY

MECHANIC
18,907 POSTS

Hi Fanning,

There are two systems that could be causing your issue. First due to the fact that the PCM is seeing more fuel (negative fuel trim) in the exhaust and commanding a lean condition means there is either more fuel getting into the exhaust or less air. Since you changed the air filter I will assume you don't have an air restriction. If you did more than likely you would have a MAF or MAP code.

However, if your PCV is stuck open then this would cause more crankcase vapor to enter into the engine again.

If this is dumping crankcase vapor into the intake then once the PCM starts reading o2 sensor data (closed loop) it starts adjusting for added fuel by removing fuel and it causes a lean condition which causes a rough idle.

The other system you need to check is the EGR system. If this is stuck open then more than likely you would have a code because this is going to distort the MAP.

If it were me, I would change the PCV and see what happens. This is cheap and easy that it is worth trying. At the very least pull it out and look a it. This is not definite but will give you a good idea of its condition.

https://www.2carpros.com/articles/engine-surges

Let me know what you find with this and we can go from there. Thanks

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Monday, June 10th, 2019 AT 4:44 PM

FANNING88

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14 POSTS

Thanks for the reply. Sorry I forgot to mention I had changed the EGR valve to because it was all gunked up also the intake passage was completely clogged and I cleaned it all out to. I watch the live data on my scan tool and it shows that it opens and closes as it should. Also I don't think my Buick Rendezvous has a PCV valve every time I type it in I find nothing and when I ask the guys at the parts store I was told there isn't one on the 3.5 motor. I changed the purge valve solenoid today just to do it since I had already bought a new one a few days ago and of course it didn't help any.I did notice that when I turn the ac on while at idle it kinda smooths out a little bit. Do you think it could have something to do with the 02 sensors could be bad or maybe even fuel injectors even though it has good fuel pressure. It may even be to high of pressure there is really no information for my year online anywhere and they don't sell the manuals at auto parts stores either they are only online for about $300.00 to $400.00 and I don't have that kind of money to spend on a repair manual. Also would it cause a problem that my ect on gets up to 189 degrees on a steady drive unless I am doing stop and go then it will raise up a few degrees and then if I just sit at idle it will heat up until the fans kick on over half way unless I have the A/C on it stays at 189 degrees. Sorry it is so long I just can't keep driving it 45 minutes to work every day cutting out so bad.

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Tuesday, June 11th, 2019 AT 5:10 AM

KASEKENNY

MECHANIC
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Here is what Alldata shows for the PCV valve.

It is possible that the o2s are an issue. If they are not reading properly then the PCM is adjusting the fuel off of bad info.

Your fuel pressure should be between 50-60PSI with the key on and engine off. What was your pressure when you measured it?

I do see on some chat pages where people with 3.5L cannot find the PCV but I am not sure what they would use in its place. Basically they should be using something to recycle blow by gasses or they may just be venting them on this engine. However, Alldata is usually pretty accurate.

It sounds like you have an issue with excess crankcase pressure because that is normally where the gunk comes from. You may need to do a combustion/intake cleaner. They have that spray foam stuff that you spray into the intake that just melts all that stuff away.

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Tuesday, June 11th, 2019 AT 10:42 AM

FANNING88

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14 POSTS

Here is the live data from my two 02 sensors while I was giving it some gas in park. My fuel pressure is at 58lbs. When I step into the throttle it drops a couple of PSI but not very much. I do have that PCV hose on my engine but there is only a small metal pipe that it attaches to on the valve cover nothing that I can take off. Thanks again for replying.

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Tuesday, June 11th, 2019 AT 11:30 AM

KASEKENNY

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Your fuel trims look okay. They are 1.6 in that screen shot and fuel pressure is good because it needs to be between 50 and 75 PSI.

Have you checked for vacuum leaks? When you turn the AC on, you are adding a load to the engine and it may be adding a little fuel due to the load.

here is a guide that shows how to check for a vacuum leak. Do you have a vacuum gauge? You should find the needle of the gauge is perfectly steady.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

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Tuesday, June 11th, 2019 AT 12:30 PM

FANNING88

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I have tried to find vacuum leaks a couple different times and haven't found any. My stft fuel trim usually is around -6 when I let off the throttle or when I put the car back in park. I am really confused about this problem. I always fix everything on my cars and this one is just giving me a hard time. I don't even know what direction to go from here. I bought a vacuum tester today so I guess ill try that and maybe a compression test and see what happens.

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Tuesday, June 11th, 2019 AT 1:10 PM

KASEKENNY

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Okay. When you say it goes to -6 does it stay there or is that just what it drops to when decelerating then it rebounds to around 0?

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Tuesday, June 11th, 2019 AT 1:27 PM

FANNING88

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It usually stays there unless I tap the gas. I also noticed after my car sit all day today I took the raidiator cap off and antifreeze started gushing out.

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Tuesday, June 11th, 2019 AT 1:38 PM

KASEKENNY

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So that means your o2 sensor is saying there is more fuel in the exhaust and it is command it to go lean but when you step on the accelerator which adds more air/fuel, it comes back to 0-ish?

You may be right that it could be an o2 sensor issue or leaking injector.

The coolant coming out after sitting may just be an air pocket in the line. When the coolant cools down, the air pocket expands and when you open the system the coolant gets pushed out.

It may be worth while to pull the o2 sensors and see what they look like. Are the tips covered? You can't clean them because you could damage the internal components so if it looks iffy I would replace it.

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Tuesday, June 11th, 2019 AT 3:40 PM

FANNING88

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Yes, when I give it a little throttle while driving it goes around 0 to positive 5 depending how much gas I give it. I will go ahead and pull the 02 sensors tomorrow and see what they look like. I have to take the intake manifold a whole bunch of other stuff just to get to the injectors so if the o2 sensors aren't the issue I will tear into again this week and check the injectors. I still don't understand why the hasn't been a check engine light after how bad it is running. That's another reason I am kind of thinking it is a fuel problem. Usually by now if it was anything else a code would of popped up. And I noticed it looked like some kind of liquid was burning on the bottom of the catalytic converter I could see it when I got home and looked under the car.

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Tuesday, June 11th, 2019 AT 3:50 PM

KASEKENNY

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Yeah. O2 sensor is smart to start with because if reading wrong then they can cause overfueling. Starting to sound like you are on the right path. The more we uncover the more will start making sense.

Keep me posted and we will get this figured out. Thanks

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Tuesday, June 11th, 2019 AT 4:08 PM

FANNING88

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Okay, I will let you know what I find out tomorrow. Thanks

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Tuesday, June 11th, 2019 AT 4:34 PM

FANNING88

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Well, I haven't had time to check the 02 sensors or anything else yet since it has been raining and I don't have a car. I am just getting worried it is the head gasket like a few people have told me but my heat works and the oil and antifreeze are not mixing. I did notice that the temperature will go up to about 230 when the fans kick on the the temperature will drop back down to about halfway and as soon as the fans kick off it will go back up and just keep doing that unless the A/C is on and the fans don't shut off. Could this be the head gasket? Someone else mention's the intake manifold gasket to. It has got to be exhaust gases getting into the cooling system. I am not sure if that would cause the engine to cut out at higher rpm's. He also mentioned that it could be cutting out from fuel or antifreeze getting into the spark plugs. Does this sound like something that could happen? Thanks

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Thursday, June 13th, 2019 AT 9:16 AM

KASEKENNY

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The temperature raising like this, could be a head gaskets. Compression test is the only real way to confirm this without any other signs.

The manifold gasket would be a vacuum leak as I mentioned earlier so yes, that could be the issue as well.

As for the cutting out due to coolant or fuel on the plugs, this is unlikely. I would assume this would cause a rough idle all the time and cutting out would result but I can't seem to relate all your issues to this.

I would suspect a vacuum leak or a compression issue as you stated.

Obviously, you will want to repair it as soon as you can because it will not get any better but from the fact that you don't have any common signs of a head gasket, you will be fine for a little while.

https://www.2carpros.com/articles/how-to-test-engine-compression

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Thursday, June 13th, 2019 AT 11:42 AM

FANNING88

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I started doing the compression test today but one of the spark plugs are stuck and I am pretty sure the guy that sold it to me knew about this problem to because it just spins in the threads once it backs out a little bit and another one on the opposite side of the motor has striped out threads. I tried to get the compression tester to screw in and it would not go in straight so I took a flashlight and looked in there and I can see where he cross threaded the spark plug but all the other cylinders. I was able to do were around 120 psi dry and around 150 psi with a few squirts of oil so now I am really wondering what is going on because that does not seem low to me unless one of the last two are bad and that is causing it I looked for bubbles in the radiator and saw none. Is there a way I can get that plug out and fix the threads while the motor is still in the car?

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Friday, June 14th, 2019 AT 3:19 PM

KASEKENNY

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If the threads are damaged, then you could be losing compression through the plug. We definitely need get the plug out, fix the threads, then test the compression.

You can fix the threads of a spark using a helicoil but doing it on the vehicle is dangerous because you have to cut out the old threads and this creates metal shavings and they will fall into the engine. Metal in the combustion chamber like this is not good. Some people have techniques of preventing this but I would not attempt them because the consequence of not getting all the shavings is a potentially failed engine.

Even if this is not the cause of your current issue, this needs to be fixed and then retest the operation of the engine. The reason this needs to be fixed first is, it could be causing your issue, and cross threaded spark plugs are only going to get worse and harder to get out.

Assuming they are not causing your issue, if not addressed now then you may be looking at a head replacement in the future.

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Sunday, June 16th, 2019 AT 9:52 AM

FANNING88

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Okay, I had my dad come over and he fixed the spark plug problem for me. He has been a mechanic his whole life so he has been there and done that plenty of times its just hard to get his help because I live over a hour away from him and we both work. So I got the spark plugs back in and 2, 4. 6 cylinders the compression is 125 psi to about 132 psi on them I haven't got to check the back plugs yet because its so hard to even get the plug wires off I ripped the boot twice already trying. I don't know why they are so hard to get off the plug I have never had so much trouble just to pull a boot off a plug. And now after driving it only a few times over the past week the fuel trims have changed a lot now the ltft is -10 and the stft is anywhere from -2 to -6 at idle but when I am driving it goes to pretty close to 0 until I get up to about 75 mph then as I watch the fuel trim when I starts to cut out I can see the fuel trims go back negative really fast then back to about normal. For some reason I cant get any pictures to load on here to show you the fuel trims and 02 sensor readings. I don't really know anything about 02 sensors but I don't thinks it looks right bank 1 sensor 2 once warmed up will stay around.725 and sensor 1 jumps from.04 to.845. Thank you for trying to help me through this.

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Sunday, June 16th, 2019 AT 5:55 PM

KASEKENNY

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An o2 sensor is just measuring the amount of oxygen in the exhaust. When the voltage is closer to.1 volts then it is leaner. When it is closer to.9 volts then this is a rich mixture.

The PCM looks at this info and will adjust the injector pulse width to increase or decrease the amount of fuel is being delivered.

Think of o2 sensor data as a health report of what the engine is doing.

Since yours is rich, here are the main causes of rich fuel mixture:

1. Excessive Fuel pressure.
2. Leaking injector.
3. Fuel pressure regulator.
4. EVAP canister is full of fuel.
5. Fuel in crankcase.
6. PCV issue.
7. Excessive blow by.
8. ECM issue.

There are more but they get into the weeds. The other possible cause is faulty o2 sensor but it looks like your engine is actually running rich so I would think we need to go through the other items first.

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Monday, June 17th, 2019 AT 1:28 PM

FANNING88

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Okay, I will try to eliminate some of the possible causes you gave me. Do you think there could still be a problem with my fuel pressure once I am driving even though it shows 58 psi when I checked it?

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Monday, June 17th, 2019 AT 1:49 PM

KASEKENNY

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It doesn't sounds like fuel pressure is an issue. You key on and engine "off" spec is 50-60 PSI so if you measured that while driving, then it is okay.

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Tuesday, June 18th, 2019 AT 7:31 AM