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760 Symptoms caused by boost pressure being set too low

lummert

lummert

Active member
Joined
Jan 11, 2008
Location
Portland IN
Been noticing difficulty keeping my 88 765 Turbo Intercooler Wagon in overdrive under the speed of 58 mph. If there's no wind the car runs good unless there's a slight incline. Below 58 mph or a slight incline trans shifts out of overdrive.

I had previously installed a 90+ manifold and TD04-13C. This turbo failed and I had to reinstall the original -89 manifold and TD05-12b. The wastegate actuator had gotten lost so I modified the actuator from the TD04-13C to fit. My tests indicate that the boost is set at about 4.5 psi at most, but closer to 2 psi when getting on it when passing another vehicle.

My thinking is that there's just not enough boost pressure before reaching about 2500 rpm. Running 215/60R-15 tires all around. Odometer is 2.7% slow. Speedometer is right on.

My plan is to try adjusting the wastegate actuator rod for more boost (perhaps closer to 7.5 psi). Any other wastegate actuators that would be a better fit on a TD05-12b?
 
Put a boost gauge in. Crank the arm up to 8 or 9 psi. Why did you have to switch the manifold to the old crappy one? The raised area/sealing surface should be the same. I am no 760 expert, but those are the cars I look to to have the Garrett T3 0.84 A/R exhaust housing, I have pulled a few of those for high rpm bolt on power. I'm surprised you had a 12b in there.

I have a pile of wastegate actuators if you need one, but I need a pic because I have no idea what they came from anymore except for a couple 15G ones...
 
2turbotoys said:
Put a boost gauge in. Crank the arm up to 8 or 9 psi. Why did you have to switch the manifold to the old crappy one? The raised area/sealing surface should be the same. I am no 760 expert, but those are the cars I look to to have the Garrett T3 0.84 A/R exhaust housing, I have pulled a few of those for high rpm bolt on power. I'm surprised you had a 12b in there.

I have a pile of wastegate actuators if you need one, but I need a pic because I have no idea what they came from anymore except for a couple 15G ones...
Click to expand...

87-89 760 sedans had Garrett turbos. 87-89 760 wagons had Mitsubishi TD05-12b. the actuator rod for the TD05-12b is offset. It has (2) 45 degree bends about 2 inches apart. Bolt holes are similar to TD04-13c. Has adjustment on the end (which TD04-13c actuator did not have). I tried to find picture but no luck yet. As far as a boost gauge goes, I've been running a vacuum/fuel pump test gauge that will test to 7 psi on the pressure end of the scale.

Mounting bracket has these markings:
78-18810
K6T02071
7X06

Link to some pictures I located:
http://forum.ih8mud.com/threads/a-crap-a-mitsubishi-turbo-question.600932/
 
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Dammit, Lummert!

I have no clue about the automatic trans or how it shifts the OD on and off, but is it possible that with your low T, the cruise control is needing to use more throttle, and that's pulling that downshift cable more, and that's triggering downshifts at lower RPM's. Try looseining that cable.

Or talk to your redblock doctor about ways to increase your T levels, and recover some of that youthful vitality.
 
JohnMc said:
Dammit, Lummert!

I have no clue about the automatic trans or how it shifts the OD on and off, but is it possible that with your low T, the cruise control is needing to use more throttle, and that's pulling that downshift cable more, and that's triggering downshifts at lower RPM's. Try looseining that cable.

Or talk to your redblock doctor about ways to increase your T levels, and recover some of that youthful vitality.
Click to expand...
Cruise control gave up the ghost years ago to to diaphrams ripping. I unbolted the cruise vacuum unit and the seconds spare part gave up.

The part about needing more throttle due to low boost is making good since.
 
Still don't quite understand why ya didn't just switch to a 15g. If ya already had a 13c, then all that'd be left is what... a wastegate actuator and bracket... and maybe an oil return?

Anyhow, back to productive talk.... If you don't have a proper boost gauge, I wouldn't be surprised if you could find a ultra cheap OEM one. SwedishIron probably has a whole turbo cluster he could sell ya for a deal. Wouldn't have markings, but you'd be able to distinguish, zero boost, super low boost, and it pegs out at like 12psi.
 
The spring inside the actuator determines boost pressure. If you crank the arm up too tight the actuator won't be able to move enough and you'll get large boost spikes or worse. Change the actuator to an 8 psi one if you want to be conservative with the boost. ATP turbo or ebay has them for the mitsu turbos.
 
I think I made a little progress. I went yesterday to my friends junkyard and got my car inside with front up of jack stands. I adjusted the actuator and now have about 5.5 PSI.

I looked through all my friends junk but only found turbos from diesel trucks that didn't even use wastegates and had different bolt pattern. Some were huge in comparison and some were a bit smaller.

I did install a T fitting in the boost hose from the compressor to the actuator, but then plugged this extra hose as it was time to go home.

Can I install a spring to help keep the wastegate shut at lower boost pressure.

I looked up universal wastegate actuators and found listings for Dodge Cummins that have adjustment from 7-21 PSI that are relatively cheap.

As far as finding a TD04H-15G, I live in a rural area and the parts can be slim pickings.

I just found a listing for a 1995 Volvo 850 GLT in the yard at Dayton, OH LKQ. Is this the 850 with the TD04HL-15G and conical exhaust outlet flange?
 
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There used to be wastegate actuators where you can change the spring to select the boost pressure. I think Forge made one.

I believe that is the correct 15g.
 
I just saw something of interest. Turbo Master sells adjustable spring loaded actuator that replaces the boost pressure diaphragm (Volvo term is pressure box). Looks easy enough to build from the right kind of hardware. Would be adjustable without going under the car.
 
Today I tried venting the hose from the compressor to the wastegate actuator. Car runs fine, barely reaches 7 psi.

On another note I located a wastegate actuator from a blown Holset HX35W. After doing the search it apperas that this turbo was off a 6BT5.9

The wastegate actuator off this turbo is adjustable between 4 psi and 22 psi. Seems like a good candidate for adapting to the mounting bracket from the TD05-12b. I'll have to figure out the bends that are needed in the rod. The pin hole for this actuator is 7.5 mm. Not sure of the pin hole for the TD05-12b actuator.
 
If you unplugged the wastsegate signal and still struggled to make 7psi, you likely have a lazy turbo, a bleeding wastegate puck on the hot side (which allows the majority of exhaust gasses to bypass the turbine, thus not making boost), or a boost leak somewhere that isn't obvious.

We boosted my buddy's 89 245 maybe 1 or 2 years ago and it was the slowest spooling turbo ever. These cars, with the 13c at least, come on quick and hard. Like, should spin the stock 185s from a dig when boost hits. 10 psi helps too I guess..

Anyway, we swapped CBVs with my 244, no change. Checked for boost leaks, none. Swapped actuators, added MBC, no help. Swapped in a brand new genuine 13c..:cool:

If you think it's downshifting too easily, it could also be a misadjusted kickdown cable, or a mechanically mistimed engine. I have seen that before. A camshaft off only 1 tooth will make the engine work way hard with way less power.
 
Holy crap, I've been having the same problem. Screwing with things around the transmission would make it better for a short time, but it always comes back. Maybe it did jump a tooth. I may have to rip off the timing cover and have a look. I remember when it started. I was slowing down on the freeway, there was a BOOM and after that, it downshifts out of 4th too early.
 
Been driving with the boost reference hose vented through a gas filter. Car runs fine. It soon become apparent the the ignition timing was too far advanced the first time I nailed it on the highway. Heard about 3 pings then let up. My vacuum/pressure tester is limited to 7 psi and it would have gone higher if the gauge was capable. I backed the timing down a bit and no more pinging so far. Fuel mileage doesn't seem to have been with the increased boost.

I'm in the process of modifying a wastegate actuator from a 12 valve Cummins to fit the Volvo actuator bracket. I threaded the actuator nearly the entire length and then cut off 4 inches. I using 5/6 steel rod to make the 2 each 45 degree bends. The thread both ends of the rod with 5/16'-24 SAE fine thread. Using thread coupler to join the new bend rod to the cut off rod of the actuator. SAE 5/16-24 threads because that what it already uses. Been the new rood tap both ends, gut to length.

Have learned enough to use leftover parts from defective actuator to use same bent rod in building a turbo-master. I like the idea of a turbo-master spring to control the wastegate but thinking pressure could be a better option in conjunction with the pretension spring in the actuator.
 
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