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It jerks and misfires under load?

Tiny
PONTIMAN
  • MEMBER
  • 1991 PONTIAC BONNEVILLE
  • 3.8L
  • V6
  • 2WD
  • AUTOMATIC
  • 180,000 MILES
The car listed above is a 3800 v6 Series 1. First of all, the car is very well maintained.
Everything started with a bad Crankshaft Sensor (the motor suddenly went off on city and highway speeds and won't restart for a while).
I put a new Crankshaft Position Sensor on, aligned it carefully several times. It fired it up but ran badly and went off after a few seconds. I put a new harmonic balancer on, aligned the Sensor again and fired it up again. It stayed on and everything felt well for the moment, so I started driving it. Sitting in front of a red traffic light, the idle speed was still a bit high, maybe 200 RPMs and the idle was a bit rough. When accelerating you can feel a misfire. The more the motor warms up (after 10 miles) the more you can feel it. Climbing up hills is more or less impossible.
I checked the Camshaft Position Sensor and checked the magnet on the Camshaft Timing Wheel - Everything looks fine.
I put new sparks, spark plug wires, a new ICM and coil pack on. - Noting changed.
I took the wiring harness that connects the ICM with the sensors and the ECM out of a good running car. - Nothing changed.
I installed a new Mass Airflow Sensor and Idle Air Control Valve. - Nothing changed.
I checked vacuum lines for cracks. - Found nothing.
I put a new ECU in. - Nothing changed.
I sprayed some brake cleaner around the intake and listened to higher idle speeds. - Nothing happened.
I checked the catalytic converter to see if it was clogged. - No, it's 3 years old and still fine.
Checked fuel pressure by pressing down the little valve in the fuel rail next to the injectors. - There is nice pressure on it (new Fuel Pressure Valve, Fuel Filter and Fuel Pump were installed 2 years ago).
I checked engine oil for whether or unburned fuel, debris, metal flakes or any kind of smell. - Nothing special in there.

Other options are maybe the O2-Sensor and the Engine Temperature Sensor for the ECU (both came new 2 years and 20,000 miles ago).
Mabey a bad injector. I got them checked at least 2 years ago.

I'm not able to tell you anything about codes at the moment, because I don't have a test tool for the OBD and the bulb for the Check Engine Light seems to be burned. I will put in a new one to see if I can pull any codes by get it blinking with a wire clamp.

Again, the symptoms:
- Rough idle.
- Deeper sound out from the muffler and a bad unburned fuel smell.
- Higher fuel consumption (maybe 25% more).
- Jerking and misfire under load.
- Sometimes jerking and scratching sound from the belt tensioner in idle speed when it misfires.
- 200 RPMs too much in idle speed and engine warm.

As you can see, I'm running out of options. Hopefully you guys have further ideas. Any ideas are welcome.
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Sunday, August 7th, 2022 AT 3:11 AM

11 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
Hi,

If the exhaust is strong, it sounds like either the fuel pressure regulator is bad, or the engine coolant temperature sensor is sending the wrong signal.

Here is where we need to start: You indicated there is good fuel pressure. We need to determine if it is within the manufacturer's specifications. You will need a fuel pressure gauge, but most parts stores will lend one to you. Follow the directions in this link:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Also, I attached a pic below and circled the fuel pressure regulator. Remove the vacuum hose on it and see if there is any evidence of fuel being drawn into the vacuum hose. If that is good, start the engine and confirm there is an engine vacuum to the regulator via the hose.

Let me know.

Joe

See pic below.
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Sunday, August 7th, 2022 AT 10:54 PM
Tiny
PONTIMAN
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  • 15 POSTS
Hello Joe,
Many thanks for your fast reply.

I agree, the fuel pressure valve and the coolant temperature sensor for the ECU can be responsible for the cars issue. Both came new 2-3 years ago, but that doesn't have to mean anything. Parts quality got more and more bad with the past few years, espacialy when using after market parts.
However, I have ordered both from that big shop that "Rocks' ;-)
I'm located in Germany near the city of Cologne, so it will take more or less two weeks until the new parts will reach me.

I also not able to use a fuel pressure gauge from a shop here in Germany. Either they don't have one, because modern cars have fuel pressure sensors already installed and can be read with an OBD2 scan tool, or they use inline pressure gauges that can't be installed onto the cars steel made fuel rail or it don't fit onto the tire valve in the cars fuel rail. And, by the way, they don't like to give away their tools to private persons like me, just to help them go on for free.
The Germans are commonly not that nice and helpful people like you guys from the US. I allowed to say this, because I'm german ;-)

Okay, back to the cars issue: Just to get you updated. On Sunday I found some time to play arround with my Pontiac, again.
I put on a new PCV. I wasn't sure if it's still working or mabey clogged. It was a hard job to get it out, down in the corner of the intake and it toke me 3 hours to get the whole thing done. The former rubber flange had turned to stone over the years and broke into pieces when I tried to pull the PCV out. Fortunately I had also a new rubber flange in stock to get the new PCV back in place to run the car for a test drive. But it doesn't effect anything.

I will follow your instructions when my parts order has arrived and come back to you with a further report when parts have been swaped.
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Tuesday, August 9th, 2022 AT 1:25 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Thank you for the update, and you are very welcome.

Were you able to locate the fuel pressure regulator and check the vacuum hose to it? Also, if you can get your hands on a vacuum pump, see if the regulator holds a vacuum.

Let me know what you find or if you have other questions.

Take care,

Joe
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Tuesday, August 9th, 2022 AT 7:38 PM
Tiny
PONTIMAN
  • MEMBER
  • 15 POSTS
Hi Joe,

The ordered parts didn't arrive until now, but I found a new, out of the box, coolant temperature sensor in my private stock. I put it on, took the car on the road, but nothing changed.
I won't stop going on, so I took the whole fuel rail with fuel pressure valve and the 6 injectors out of my good running Bonneville and put it into the bad running one. - Again, nothing changed.
And the good running Bonneville got the parts from the bad one and still runs fine.

I also checked the vacuum on the vacuum line that runs from the top of the intake to the fuel pressure valve. There is a strong vacuum when I put my finger onto the end of the vacuum line, so everything seems to be fine.
On my way back home, I stopped my car beside the road and pulled the vacuum line out of the fuel pressure valve: the idle jumps up an additional 200 RPMs to 1,500RPMs and then some. I drove it home the last few miles with the unplugged vacuum line and it was more sportive and powerful when accelerating and climbing up hills, but the misfire and rough idle was still there, just came up a bit later regarding to the RPMs.

I wasn't able to check if there's power on the plug of the coolant temperature sensor, because I couldn't get access to it, when the engine was hot. I will check this next.

There're also a few questions, you hopefully know the answer:
What about the vacuum switch on the vapor canister on the driver's side? - What will happen if it fails?
What are the three relays for, which are located next to the air filter box?
What are the relays for, which are located next to the wiper motor?

I am just asking for the relays, because I had an issue a few years ago with that car: in summertime, with A/C on, the interior fan suddenly stopped working when the engine warmed up. It took a while, until I figured out, that a few soft knocks on the fan relays got it back working for a few minutes. During wintertime, the issue was gone. When the summer came back, also the issue came back. I pulled out those fan relays, which are located in the passenger side engine bay below the windscreen, and put new relays on, and the issue was finally solved.

Mabey the relays are worn within the past 30 years and suddenly stop working when the engine bay heats up. - What do you think?
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Sunday, August 14th, 2022 AT 1:46 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
Hi,

It's interesting it ran better with the fuel pressure regulator disconnected. The rough idle was likely the same or worse.

Are you still getting a fuel odor from the exhaust? Have you checked the catalytic converter to see if it is partially plugged?

Take a look through this link and let me know if anything mirrors what you experience:

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

Also, if you haven't already, check to see if there are any diagnostic trouble codes stored. All you need to retrieve them is a short jumper wire or even a paper clip. Follow the directions in this link and let me know what you find.

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Also, I attached a pic below showing the relay box.

Let me know what you find or if you have other questions.

Take care,

Joe

See pics below. Pic 1 shows the location and pic 2 identifies the relays.
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Sunday, August 14th, 2022 AT 7:49 PM
Tiny
PONTIMAN
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The rough idle was just a bit worse. I'm sure it was just because of the 200 RPMs higher idle speed.
The smell from the exhaust isn't just a fuel smell and it's also not the smell like rotten eggs from bad fuel. It smells more poisonous.
I had a look at the spark plugs. They look all the same and they are a bit white powdered like they look if there is a lean mixture. They were all dry. I'm just wondering, because I would have expected that they would be black powdered and wet from a rich mixture.

The catalytic converter isn't clogged. I took it out and all those honeycombs are free. There is also no rattle inside or loose parts that came out, if you shake it. The muffler and tube are also not clogged. I put the whole exhaust out and let it run. It just became horrible loud like a drag racer, but the rough idle and misfire is still there.

Thanks for the picture of the relays box. I already know that one. It is the relays box behind the kick panel on the passenger's side.
I'm more interested in the relays in the engine bay, next to the air filter box. There're three relays in a row mounted to sheet metal. It's nothing subsequently installed, because both of my Bonneville's have this the same.

I will come back to you at the weekend, when I've pulled the codes.
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Monday, August 15th, 2022 AT 12:58 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
Hi,

As far as the relays are concerned, they are for the cooling fan. See pic below.

By chance, does the odor from the exhaust have a sweet smell?

Let me know.

Joe

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Monday, August 15th, 2022 AT 7:44 PM
Tiny
PONTIMAN
  • MEMBER
  • 15 POSTS
Hello Joe,

Many thanks for the information regarding the relays.

The odor from the exhaust doesn't have a sweet smell. I don't have any loss of coolant fluid, so the head gasket seems to be fine.
A friend has a tool where you can put in some blue indicator fluid and hold it to the outlet of the muffler and let the car run for a few minutes to let the emission pass the indicator fluid. If there is coolant burned in the engine, the indicator fluid will change its color.
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Wednesday, August 17th, 2022 AT 5:52 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
Hi,

At this point, it may be a good idea to check engine compression. Based on mileage, it could have a weak cylinder. I will say that the intake manifolds are known to leak. Are you certain there are no engine vacuum leaks?

Here is the link for checking engine compression:

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

Here is a link that explains how to find and repair a vacuum leak:

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

Let me know.

Joe
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Thursday, August 18th, 2022 AT 6:39 PM
Tiny
PONTIMAN
  • MEMBER
  • 15 POSTS
Hello Joe,

yesterday, I went on with my car, I checked the coolant temperature sensor wiring. It gave 5V to the sensor input and the sensor gives out the same in cold condition. When runing the car and the coolant temperature increase, the voltage goes down, so the sensor works and the wiring to the ECU is fine.

I replaced the SES light bulb, now the light comes on, if you turn the key is in position 2, as normal. I started the car and the SES light went off as normal.
I took the car on the road for 50 miles and no SES light came on.

Today I drove the car again, but more in race style and also climbed up some hills.
Suddently the SES came on, stayed for a while and went off again. This happened a few times randomly at highway and city speeds, without any change to its runing conditions.
I stopped beside the road an put a wire clamb on to read the codes.
I got a:
#12 - OBD test mode
#26 - Quad Driver Module (QDM-A)
#34 - MAP Sensor Circuit Error (signal low indicating high vacuum)

You remember, I told you there's a high vacuum and I pulled the vacuum line and the car starts later to stumble and missfire.
However, the code #34 isn't important to me, because I put on a new MAP sensor and nothing changed.
We have to check the code #26, I don't know, what the QDM-A is and where to find it. It's not part of the ECU, because I put a new in before and nothing changed.
I guess it could be a faulty EGR valve, or bad wiring to EGR. - What do you think?
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Sunday, August 21st, 2022 AT 9:48 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
Hi,

The 34 indicates a low voltage. Keep in mind that it can be the result of the sensor or a wiring/connector issue.

As far as the quad driver, that is internal in the ECM. Each module controls a different circuit.

I attached the diagnostics below. Note, there were three pages. I included what I thought would be relevant for you.

Let me know your thoughts.

Joe
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Sunday, August 21st, 2022 AT 5:36 PM

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