'94 850 Turbo-Feels Low on Power

[IG-88A]

So, I've had this car for about two years now. Its been good to me, for the most part, aside from some maintenance and some random component failures here and there. But it seems as though I was never fully aware of how it was supposed to feel, power-wise. Oddly, last week, for two or three days, it felt as though it had come alive out of the blue. The throttle was snappy, it felt like it was breathing deeply, and I was having a great experience. The "joy" of torque steer is something I hadn't yet experienced in this car, until those few days came. Same with the joy of being pushed back in my seat. Today, its back to its old self again. The old self that I was completely comfortable with, until I learned of its full potential. Now I want that potential back. But I don't even know where to start looking for the source of what's holding it back. I was hoping that some of you might have some suggestions. Parts that I've replaced that would affect such behavior are cap, rotor, plugs, wires. Plugs are Volvo, gapped properly. Wires are Bougicord. Fuel filter is new. Coolant temp. sensor is new. Thermostat is new. Vacuum lines and turbo hoses are new. IAC hose is new. Fuel pump relay is new. Fuel tank is new-ish. Air filter is new. The odd thing is, this spark of life happened the day after an oil change, which should make no difference. I want this 850 that I've been missing all this time. I'll add that I'm getting the same old EVAP code that I can't find the source of to save my life, and a front knock sensor code that I also can't solve. But They usually return only once every couple of weeks. They were present during this good period. I cleared them, and the good continued. Yet today they're back, and so is the sluggishness that I didn't realize was sluggishness. Any clues would be appreciated. Thanks.

[pkc303]

Check your vacuum lines again. My 95 turbo had a vacuum line loose, and I fixed it to find the power return. Sometimes they are hard to find, the most common may be the one to the fuel regulator under the fuel rail on the pax side.

[j-dawg]

Agreed, check for leaks.

Unplug your battery overnight and let the ECU reset itself.

Scan for any error codes.

Are you pumping in premium fuel? You should be. If the car detects knock due to lower-octane fuel, it may retard the spark, which will impact power and gas mileage.

[rspi]

I had the same issues for a while. The tale tale sign is your boost gauge. The boost gauge tells you where the computer is allowing the system to boost to. The BCS will open the waste gate actuator and slow down the speed of the turbo, bleeding off your boost pressure. Taking the FUN right out of DRIVING.

Why you may ask... in the name of protecting the "system" from causing more damage to whatever is wrong with the car. Emissions, sensors, whatever. It is kind of a semi-limp mode. Sucks! So instead of boosting at 9 psi, it will dial back the boost to 2 or 3 psi. Enough boost to let the car feel like a good running NA, but by no means powerful and fun.

When I reset my codes with my OBD2 tool, I'd get my power back for a drive cycle or two, until the pending code would alert the ECU that something wasn't right.

Down to business and the solution... My car had a weak o2 sensor. It also has a undersized cat. The nice thing to me was that I had my ScanGauge connected to my OBD2 port all the time. When I seen that my boost was less than 1/2 of the white area, I would preform a force reset of codes that were pending or not there and get my full boost back for a while. I also had a spare "tuned/chipped" ECU that would ignore the laden pending code. My main code was a P0422.

The other issue I had was the fact that my turbo parts were old and wore out. So, when I bypassed my mild limp mode, and gained my R power back, it was short lived, lasted about 2 weeks before my bypass valve on my turbo gave up and tore, leaving me with 1 psi of boost. That went on for a couple of weeks until I figured that out. So I replaced the bypass valve. I also replaced the waste gate actuator in the process.

Well, since I had an undersized cat, I ran the chipped ECU until I got replaced. I also replaced my 15g with 293,000 miles on it for a 16t with about 130,000 miles and my downpipe to a 3". My old turbo was doing fine but was slowing down and would not hold 10 lbs of boost, was only holding 8. I changed the turbo, mainly because the other exhaust work was being done.

Hope that helps.

[abscate]

That knock sensor code is telling you something is wrong enough with the engine that it can't control the timing to stop precognition. It is NOT the knock sensors themselves, 99.99% of the time.

Once engine management is unhappy with timing, boost is inhibited and you get the Detuned experience.

How old is the turbo control valve by the way?

[rspi]

This video shows low boost, about 3 psi.

[rspi]

[IG-88A]

Check your vacuum lines again.

I feel like I'm always going over these lines with a fine-toothed comb. I'll check that one to the fuel pressure Regulator, and go over the rest again. Thanks.

Are you pumping in premium fuel? You should be. If the car detects knock due to lower-octane fuel, it may retard the spark, which will impact power and gas mileage.

Yes, I always used premium fuel in this one. I've never strayed. I should have mentioned that in my post. Thanks. I'll try resetting the ECU, and note the car's behavior. Those two codes are the only ones that ever show up.

That knock sensor code is telling you something is wrong enough with the engine that it can't control the timing to stop precognition. It is NOT the knock sensors themselves, 99.99% of the time.

Once engine management is unhappy with timing, boost is inhibited and you get the Detuned experience.

How old is the turbo control valve by the way?

Thanks.The TCV/BCS is as old as the car, as far as I know. I know I've never replaced it, and didn't see any mention of it in the previous owner's notes in the owner's manual, or in his receipt collection. I have one or two in a box of spare parts I'd happen upon at the junkyards. I'll try one and see if that helps. So if the 143 blink code isn't coming from a bad sensor/bad connector/etc., and I'm using premium fuel always, what should I be looking at as the source of it? I always seem to read sensors and fuel as the main faults. But I've never seen much more. The only other cause I've seen listed is carbon buildup on top of the pistons. This thing had some when I changed the plugs. But I don't know what can be done about it, except to wait and hope that with regular driving and good fuel, it eventually goes away. In fact, when I did the PCV the intake was caked with it. I'm hesitant to have the dealer look at the KS problem because it seems they always like to try to sell people a new ECU.

I had the same issues for a while. The tale tale sign is your boost gauge.

Thanks, rspi. My boost gauge seems to behave correctly, but maybe boost is a little weak. when coasting, its usually around the 1/4 mark. And under WOT, it seems to be around the 10 lbs. area you describe in your video, about halfway up the white, maybe even to the 3/4 mark at times. By your underboost/overboost description, it seems I should be in the range of being okay, I guess. I have a scan gauge too. I recently picked it up, and I've not gotten into it too deeply. Is there anything that I can monitor live on the scan gauge that can help me pinpoint the source? I should probably install a proper boost gauge, so I can see what the car is actually producing. I have a new turbo back exhaust on the way very soon, (just waiting for that tax refund to clear the bank) as mine has a small leak around the cat that's been welded and re-welded. I'll likely replace the O2 sensors during this process. Due to this new exhaust being set up to accept an angle flange TD04, I've also acquired a Angle flange exhaust housing from a 16t, and will be using that on a freshly rebuilt 15g that I just got back a couple of weeks ago. I'll replace the CBV and the wastegate actuator during that process. I'm hoping this will cure some of its ills. Thanks as well for the videos. I've watched many of your videos before doing jobs the last couple of years. Its a great service you provide.


Thanks to everyone who replied. For starters, I'll try out one of the spare TCVs today, and see if that makes any difference. Any other advice would be appreciated. I'll be sure to update this if I find anything later today.

[Volvofan94]

My boost gauge seems to behave correctly, but maybe boost is a little weak. when coasting, its usually around the 1/4 mark.

I'm wondering if the metal tube that runs on the top of the valve cover needs a good cleaning. I noticed this problem with my car (also a '94 850 Turbo) and when coasting down a hill, the boost gauge wouldn't go past the 1/4 mark on the vacuum side. If things were good the boost gauge would hover around the 1/8th mark when coasting. After a thorough cleaning performance returned to normal. Something to try if it hasn't been cleaned for a while.

I asked about the metal tube / boost performance in this thread below.
https://www.matthewsvolvosite.com/forums ... =1&t=62014

-volvofan94

[IG-88A]

I'm wondering if the metal tube that runs on the top of the valve cover needs a good cleaning.

Thanks Volvofan. That thing definitely needed a good cleaning at one point. I tried to get it clear, but never succeeded. I just ended up bypassing it with a length of vacuum hose, and eventually removing it. So you're saying while coasting, my needle should read at about 1/8? Interesting. I'll be looking into things. I'll let you know if anything seems clogged up with gunk.