Loss of power after driving at low rpm's

Hi,

It seems you have a couple different issues. The idea that the idle searches as you described is usually the result of a failing idle air control valve. Now, since the check engine light is usually on, I strongly recommend having the computer scanned for diagnostic trouble codes. They will most likely point us in the right direction of the problems.

Here is a quick video showing how it is done:

https://youtu.be/YV3TRZwer8k

I realize that most people don't own a scanner, but often times a parts store will do it free of charge or lend/rent you a scanner. This step really needs done.

Now the power loss after low RPM basically sounds like the engine is loading up with too much fuel. The idea that the idle also can be an issue indicates to me that maybe there is a blockage to the air inlet between the air filter box and throttle body, so check that.

Next, high fuel pressure due to a faulty pressure regulator can cause the fuel mixture to be too rich. That can again cause loading up of fuel at low RPM's.

A partially plugged catalytic converter can also cause issues.

Rather than giving you fifty things to check, what I need you to do is have the computer scanned and let me know what diagnostic trouble codes were present. That will save a lot of time and get us focused in the right area. If that is something you can't have done, let me know.

Take care and let me know.

Joe

Hi Joe,

Thanks for your answer and sorry for taking so long to reply. I took a while to get a DiCE unit and the correct VIDA software plus a laptop, but now I have finally scanned the car. The computer errors I'm getting have nothing to do with the problem. (One is a faulty signal from the drivers mirror, and the other one concerns the alarm unit).

I also replaced the Oil Trap and the engine breather tubes (PCV), because they needed to be done, but the problem still persists...

The old oil trap was 50% clogged, and that seems to have gotten rid of the erratic idle...

I'm not sure about the loading up of fuel... I don't smell extra fuel, there's no smoke from behind (even when car is cold), and the feeling I get is that the Turbo doesn't engage, the engine sounds a bit louder and struggles to build up RPM's. With fuel load could the turbo fail to engage?

If it were a clogged catalytic converter, wouldn't the power loss would be permanent? (happened to me in my old 850 4 years ago - cat melted and clogged half the passageway). Sometimes the car runs fine. Actually, it runs fine most of the time, as long as I'm pushing it a bit (above 3,000 rpm's).

Sometimes, when the power is low and doesn't want to build up, if I shift to second gear and really punch it, it will build up rpm's very slowly for a while and then suddenly release, and everything goes back to normal again, the turbo kicks in and the car gains impressive life and speed. It's a beast above the 5000 rpm range.
I see how the fuel build-up makes sense here, and the fact that I'm forcing it up to higher rpm's could force the engine to burn or expel the extra fuel, but I don't understand where it could be building up... I recently took the fuel rail apart to do the PCV job... The fuel is either before or after the injectors. In the fuel rail or in the cylinders.... For fuel to build up in a cylinder, that would mean one of the sparks isn't firing, right? Could that affect the turbo and account for the louder sound?

With these new conditions (better idle, PCV kit changed and irrelevant error codes), any idea what it could be?


Thank you very much

Hi,

You know, I read and reread what you wrote. I can't be sure, but I have a feeling it is related to a partially plugged catalytic converter. What can happen is the internal guts of the converter can come loose and move around causing partial blockage at different times. The idea that it seems louder can be a symptom, and the idea that the turbo doesn't seem to work correctly can be a symptom. If the exhaust flow is blocked, that is what actuates the turbo. Have you tried removing an upstream o2 sensor to see if it makes any difference?

Take a look through these links. The first one describes common symptoms related to catalytic converter issues and the second is how to check a converter. The second one indicates to remove the converter. However, if you remove an oxygen sensor before the converter, it will make a difference that is noticeable. It won't be perfect because the exhaust is still restricted, but you should notice a difference.

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

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

If possible, just try that and let me know if it makes any difference. Since there are no concrete codes to go by, we need to start eliminating possible causes.

I will watch for your reply. I hope you and your family is well.

Take care of yourself.

Joe

Hi Joe,

Thank you for that detailed answer. I have read both the links you sent me, and I have to say I still don't see it. I'm a computer programmer, so logic and identifiable patterns rule my life. If the problem is in the catalytic converter, then there must be a pattern to be found. So far, the only pattern I have is that driving for a few minutes under 2,000 rpm's always triggers the problem. No bumps, no sharp corners, simply driving at low speeds.

So I went out and did a few tests:
First, I got under the car with a rubber hammer and tapped the converter a few times to see if I could identify any loose parts knocking about inside it. I couldn't.
Then I drove it full speed uphill (idea was to force it into maximum effort). It just flew up the hill as if nothing was wrong (what a joy!). Then I drove around for a few miles at low rpm to trigger the loss of power and tried to get up the same hill.... It barely made it, at a top speed of 20 mph (earlier I reached the top of the hill at almost 10 0mph).

Then I stopped the car and waited 10 minutes. When I started it was back to normal. I let it idle for 5 minutes and it triggered the problem again. So slow driving and idling both trigger the problem. But driving fast, always keeping revs above 3000 never triggers it.

For it to be the converter, according to your first link, and because I don't get any error codes, it could only be if a piece had broken off and was randomly causing this loss of power. This does not seem to be the case. There's nothing random here since I can replicate the problem every time I want.

The fuel build up seems more plausible in this case, but as I said before, no smoke, no excessive fuel smell, no backfiring (in my mind, if fuel was building up in a cylinder and then released, I should get something similar to a backfire)...

I also posted this question on matthew's volvo site, and I'm being advised it could be the Turbo Control Valve. Does this make sense to you?

I'm a smart-ish (in a problem-solving kind of way) and very logical guy, and have always done my own mechanics ever since I got my first car 28 years ago, but I am by no means a mechanic. I manage to do this by googling the symptoms, checking out all the answers and then doing a few tests and using logic to identify the problem and not being afraid to take things apart and look at them. In this case, according to what I wrote, the TCV could make sense, but I feel that's an answer based on the fact that I mentioned the Turbo wasn't engaging. This may not be true. It feels like the turbo isn't engaging, but I can't be sure. Maybe the turbo is working fine all the time and the problem is somewhere else.

In my mind, if the Turbo does not engage, I'll have less air in the chambers. With less air flowing through, the MAF will report back and tell the ECU to cut down on the fuel (right?). So the symptoms of a non-working Turbo would be similar to not pressing the accelerator pedal. This feels like what is happening. Even when I floor it, the speed builds up really slowly, as if I'm accelerating as gently and as slowly as possible, until it reaches a point where it seems the car is too heavy to be able to build up more speed (around 20mph when going up that hill). If I go downhill, the sheer weight of the car helps it build up more speed, but also very slowly.

My car does not have a turbo boost gauge, so I have set up and ELM127 OBD2 reader and installed Torque Pro on my phone. I will drive it around later while monitoring the turbo pressure. Also going to try and test the TCV by bypassing or blowing through one of the lines (as described here: https://www.matthewsvolvosite.com/forums/viewtopic.php?t=63924).

I will report back with my findings.

Thank you again, Joe.
Stay safe.

Simon

Hi,

Can you provide me with the short term fuel trims? I didn't realize you had the scanner. Also, take a look at the picture below. This is the only option my manual is giving for a V70. Is this correct? Also, let me know the results.

Let me know.

Joe

Hi Joe,
No, that car is not correct. Mine is as listed above, Volvo V70 T5, 2,3 Litre Petrol, Turbo (184KW - 250HP).

I'll pop down later with my scanner and get those readings.

Thanks,

Simon

Here's the Info I just scanned at idle and car warmed up to normal operating temperature (see attached picture).
The first value oscillates between 0.99 and 1.02, all the others seem to remain the same.
Not sure why short bank 2 does not give a reading... Do I not have a bank 2 or does this indicate something wrong? (I had to google short term fuel trims, had never heard of this before).

Thanks Joe.

Hi,

The STFT in a perfect world would be 0. The idea that this shows a 1.02 indicates the computer is adding fuel to maintain the idle. If it was a negative number, the opposite would happen. So, are you certain there are no vacuum leaks? Fuel pressure is good and so on? Anything that can cause the computer to be adding more than normal fuel to the mixture?

Joe