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Discussion Starter · #1 ·
I've been chasing down a loss in turbo pressure for months now and am fairly sure it's an engine management problem - something is telling the BPC to open the wastegate too soon. One problem with finding the culprit is I'm not getting any codes. Would a Tech 2 be able to give more info on specifics?
 

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It'll can be:

A. the BPC itself,

B. a bad wire going to or from the BPC,

C. a bad vacuum line,

D. maybe a rip in the wastegate diaphragm causing troubles with the wastegate opening/closing correctly

Disconnect the wastegate arm, then tie the waste gate shut. If done correctly it should boost until it hits fuel cut. If that works consistently (it should) without boost loss, then reroute the vacuum line from "T" (turbo) and "W" (wastegate nipple) on the turbo so they are connected to each other. Leave the source vacuum line on the BPC alone, and stick a small section of vacuum hose connecting the 2 open BPC vacuum ports together. Take for a drive. It should boost until the wastegate opens which I think is like 6-8 PSI. This will basically be bypassing the BPC and running off the WG's spring. You MUST have a boost gauge with NUMBERS hooked up to the car so we know what's going on. If you aren't seeing 6-8 psi (whatever the spring is rated at) then expect a bad diaphragm in the wastegate. You can also attach a hand pump (with a gauge attached to it) to the wastegate nipple on the canister and see if it's holding any pressure at all or if it's leaking. Should be pretty apparent if it's leaking. If everything checks out so far, move on the BPC. It should be making noise when the car is running. Make sure that wiggling wires doesn't cause it to cut out. Make sure ALL the rubber hoses attached are NEW as they can rot in one spot and look new everywhere else. Make sure you have NOT altered the signal line that runs to the TB to run from somewhere else. Make SURE the "W" goes to wastegate, and the "T" goes to the turbo's pressure side. If all is OK so far, replace the BPC and see what happens.

I've covered everything to check. It'll be one of those things. The ECM *can* limit boost if there's a reason to, but if it's limiting boost so severely that the car isn't performing correctly, then something with the engine or DIC, plugs, timing, etc. is wrong.
 

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Discussion Starter · #3 ·
It'll can be:

A. the BPC itself,

B. a bad wire going to or from the BPC,

C. a bad vacuum line,

D. maybe a rip in the wastegate diaphragm causing troubles with the wastegate opening/closing correctly

Disconnect the wastegate arm, then tie the waste gate shut. If done correctly it should boost until it hits fuel cut. If that works consistently (it should) without boost loss, then reroute the vacuum line from "T" (turbo) and "W" (wastegate nipple) on the turbo so they are connected to each other. Leave the source vacuum line on the BPC alone, and stick a small section of vacuum hose connecting the 2 open BPC vacuum ports together. Take for a drive. It should boost until the wastegate opens which I think is like 6-8 PSI. This will basically be bypassing the BPC and running off the WG's spring. You MUST have a boost gauge with NUMBERS hooked up to the car so we know what's going on. If you aren't seeing 6-8 psi (whatever the spring is rated at) then expect a bad diaphragm in the wastegate. You can also attach a hand pump (with a gauge attached to it) to the wastegate nipple on the canister and see if it's holding any pressure at all or if it's leaking. Should be pretty apparent if it's leaking. If everything checks out so far, move on the BPC. It should be making noise when the car is running. Make sure that wiggling wires doesn't cause it to cut out. Make sure ALL the rubber hoses attached are NEW as they can rot in one spot and look new everywhere else. Make sure you have NOT altered the signal line that runs to the TB to run from somewhere else. Make SURE the "W" goes to wastegate, and the "T" goes to the turbo's pressure side. If all is OK so far, replace the BPC and see what happens.

I've covered everything to check. It'll be one of those things. The ECM *can* limit boost if there's a reason to, but if it's limiting boost so severely that the car isn't performing correctly, then something with the engine or DIC, plugs, timing, etc. is wrong.
Way ahead of ya....
New vac lines
New BPC
New DIC
New plugs
Checked for leaks everywhere
I get full boost to fuel cutoff with the wastegate vac line disconnected. Car runs great besides the base boost problem.

Thanks Pontius
 

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What resistances are you seeing between the pins on the BPC valve? Should be about 5 ohms from pins 1 to 3, and 2.5 ohms between pins 1 and 2 and between pins 2 and 3. You said that you replaced the BPC/APC valve already; what was wrong with the old one? Have you checked the 'APC' fuse?
 

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Discussion Starter · #5 ·
What resistances are you seeing between the pins on the BPC valve? Should be about 5 ohms from pins 1 to 3, and 2.5 ohms between pins 1 and 2 and between pins 2 and 3. You said that you replaced the BPC/APC valve already; what was wrong with the old one? Have you checked the 'APC' fuse?
It's a T7, only 2 pins on this BPC. Think it is about 3.5 OHMS showing on the old and new one. Getting ~12v at the harness. I haven't found an APC fuse yet, gimme a hint where it is.
 

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I expect you'd find the APC fuse in the fuse panel on the side of the dashboard that is covered when the driver-side door is closed.
 

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Discussion Starter · #7 ·
I expect you'd find the APC fuse in the fuse panel on the side of the dashboard that is covered when the driver-side door is closed.
Checked everything labeled as dealing with engine management - all good.

What do the fuses at the front US drivers side by the battery control?
 

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Checked everything labeled as dealing with engine management - all good.

What do the fuses at the front US drivers side by the battery control?
The 60A fuses near the battery in the front-left area under the hood should feed to the ignition switch, passenger fusebox, fuel pump relay, lighting switch, starter relay, and main relay, at least they do on the 1994-1999 900/9-3 according to this Hayes manual...
 

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Discussion Starter · #10 ·
Thx for the replies, I appreciate it...but I want to get back to the original question. Can a Tech 2 give me more info about engine management? Maybe it can pick up something the the ECM wouldn't throw a code for?
 
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