I read the 700/900 faq's, and did not see anything like this for my '89 760.

At 55 on faily level road all is pretty smooth - turbo gauge stays put at about 1/4.

Push it a bit harder going around traffic or up a hill, and turbo kicks in - good punch - but then will drop out, just for sec, and then pick back up. At other times when you push it, it will speed up - like with extra boost - then back to normal boost.

Cruise at 70-75 with cruise on and turbo will engage erratically like someone who would pump the gas - then let off. Then it may go for a spell just as smooth as can be.

I hope someone has some ideas. I just spent over 500.00 at the shop. This was the one thing that he has not figured out yet.
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'89 764 (110K), '94 940T (180K) , '92 745T (totaled 2-10-05 RIP)

I believe there is a sensor mounted on the outside of the coolant reservour.
It has a vacuum hose attached to it and an electrical connector. This is an overboost pressure switch. If the boost spikes over 14.9 psi it shuts off the fuel. Perhaps this sensor and its vacuum line is something to check out also.

fyi..

--
dnvolvo '89 765T - '91 245

This is just a wild guess here, but have you checked that wastegate actuator hose? I purchased an 1988 760 Turbo 10 days ago and my actuator hose was completely cut off right underneath it's clamp. This caused massive overboost if I stepped on the gas and around 75-80 mph the engine wasn't pulling steady, it wanted to "surge" a little bit. Haven't tried it on the highway after fixing the hose, but it's a LOT better now around town.


-M.R.

Thanks for the reply.

OK..I found the wastegate hose (about 6" long - going down). i will check that. It is not easy to remove (non-screw clamps) and it looks new. It is not really touching the side of the turbo. I will get it off though.

While I was pondering at the car, I noted there is a silver colored device just above the water pump , in front of the #1 cyl. It looks like a large vacumn device. Out of the bottom of this is a good size hose (about 1 1/2 - 2" dia) that connects to the main intake from the airbox to the turbo. There is a single vacumn line that connects from the top of this to the intake manifold.

1. The curved hose from the device to the large intake is somewhat soft on the bottom of it's curve, and up the side a bit.
2. The vac hose has a small red device in the middle - with arrows on it indicating that the flow of air be toward the intake. I disconnected it from the device above the water pump, and blew through it - no resistence. I then took it off on both ends - took off one half of the hose (toward the turbo) and blew into the red device (stopping the other end). When I blow, I feel air coming back toward me. When I suck the air - the is a bit more resistence, but air still moves freely - I just don't feel it as when I blew.

Have I found something - I hope, I pray.
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'89 764 (110K), '94 940T (180K) , '92 745T (totaled 2-10-05 RIP)

I believe it is the external blow off valve. Your Mitsub. had and internal one.
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90 744ti, Airbox, K/N, Cold air, G-Valve, Dual boost control, VDO, Modified exhaust. 88 745 gle.

The red/gray colored plastic valve - I believe - to be in real Volvo terms to be a "delay valve" there should be resistance when you blow thru it both ways, this is in the 700/900 FAQs.

The blow off valve is mounted near the timing belt cover on the passenger side of the engine and is metal, it connects to the S Shaped AMM to Turbo hose.


If I understand correctly, this is the right nomenclature for the components.

The delay vaves are really cheap, I have a couple if you need one. It has membranes in it and they can wear out after while. If you pay me 5.00 I'll ship you one, basically the cost of shipping, only if you want to.

Also, make sure you replace the hose from the Turbo to the Wastegate actuator diaphragm, it takes like 5 minutes to replace this hose, do it while you are there.


Also, do you hear a hissing when you are having this performance issue ? If you are hearing the hissing, then you have a leak somewhere on the pressure side of the turbo system. You need to find it, it seems to be only happening when it's under load, so it may be difficult to recreate the problem without duct taping an ex wife with a digital camera under the car while you drive around and put a load on the motor.

I will probably take you up on that valve - thanks. I will let you know when.

Hissing ...I THINK I have heard this - but I need to listen closer,a nd make sure it is not the valve train.

If it is a leak on the pressure side - it almost has to be a hose - right?

Thanks so much for this.

By the way - driving just a while ago today on the freeway at 70 - the car loses power - seems to stay running - but eventaully the lights come on the dash. I am by this time doing about 25. I put it in neutral, and it cranks right back up. I wonder if all this ties in with the problem?

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'89 764 (110K), '94 940T (180K) , '92 745T (totaled 2-10-05 RIP)

>At 55 on faily level road all is pretty smooth - turbo gauge stays put at about 1/4.

1/4 what?

>Push it a bit harder going around traffic or up a hill, and turbo kicks in - >good punch - but then will drop out, just for sec, and then pick back up. At >other times when you push it, it will speed up - like with extra boost - then ?>back to normal boost.

Boost gauge is doing what during all this?

>At 55 on faily level road all is pretty smooth - turbo gauge stays put at about 1/4.

I am considering half as straight up ...\|/ ..3/4 would be into the right.

>Push it a bit harder going around traffic or up a hill, and turbo kicks in - >good punch - but then will drop out, just for sec, and then pick back up. At >other times when you push it, it will speed up - like with extra boost - then ?>back to normal boost.

Boost gauge is doing what during all this?
It kicks over to the right ...3/4 or better - then will drop a bit with the drop out. This like during a pass.

The other where I am trying to cruise at 70-75 - it will usually be a bit below half (when it is smooth), and then jump to the right - just like I had blipped the gas (but I did not).

Hope this helps.
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'89 764 (110K), '94 940T (180K) , '92 745T (totaled 2-10-05 RIP)

I agree; it sounds like a wastegate problem. Check the linkage under the manifold, too.
The device in front of the water pump is the compressor bypass vave, the plastic do-dad is a check valve.

I wonder if this could have anything to do with with boost pressure. Coming out of the gate - when I give it a bit of a push - it really will take off - more so than my 745t ever did. Just wonder if too much boost could cause this type of problem.

I am not sure how to check the wastegate. If it is where I think it is - it is realy hidden and close - have to wait for it to cool.

It is all underneath - right?

Man - I really apprecaite the help.

Side note:
I went to bed last night, murmering and complaining (to myself) - saying that I would just sell the car. Got up this AM - and the first thing that came out of my 4 year old grand-daughters mouth as she woke ..
"Poppa, you drive that car". Now that is something! I had not really said anything to her.
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'89 764 (110K), '94 940T (180K) , '92 745T (totaled 2-10-05 RIP)

May or may not be relavent. The gas gauge is mostly not working - will come on every once in a spell.

The mecahnic that worked on the car all day yesterday said that his next place to look was in the tank sending unit area for a hose that was torn - creating a vaccumn in the cavity. His recommnedation was that I fill the tank and see if it runs any better with a full tank. I will do this today and post back.

Actually, this may be part of fixing the gauge. We will see.

Anyone heard of this?
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'89 764 (110K), '94 940T (180K) , '92 745T (totaled 2-10-05 RIP)