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Boosting properly?

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Old 06-14-18, 04:33 PM
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Smile Boosting properly?

Hey Everyone,

I just want to say thanks to anyone in advance that takes the time to respond to me here. I know I can find what I am looking for but I would like to get this figured out sooner than later. I just posted in the new members section as I am just returning after being gone for like almost 10 years. I am looking at a low mileage Silver FD touring 93 model that looks impeccable. I took a brief ride with the owner last Saturday before he had to leave town. The car sounded and felt great, idled great, hardly any smoke after sitting for a couple of weeks on a trickle charger and without any metering all would seem well. Towards the end after several attempts to push the turbos I noticed the aftermarket boost gauge was reading only about 5 psi consistently through gears even mashing it on the highway, and at idle I believe it was sitting in the negative (cant recall if this is normal or it should be at 0).

1) Could there be a vacuum hose leak causing the boost to be weak? I seem to remember this being a possible problem with my turbos in my 93 touring back in the day and it was the first thing I totally replaced on the car.
2) I forget what connections are made for the boost gauge, could it be improperly connected and showing less boost than there is?
3) What is the easiest way for me to verify the boost pressure? I do not have any of my old parts like Greddy boost controller or Apexi Power FC or anything.
4) Anyone in Colorado reading this want to check her out with me? =D
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Old 06-18-18, 12:11 AM
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A vacuum leak going to any of the boost/wastegate control solenoids or reservoirs could cause a loss of boost. My vacuum chamber behind my alternator was cracked for the longest time and I was down on boost, couldn't figure out why until I swapped out my PS pump.

At idle, your gauge should be reading vacuum (so, negative). Mine runs about -0.65kPa, so, whatever that is (I think like 11 mmHg or something?). The boost gauge is typically plumbed to one of the two nipples on the UIM next to the brake booster vacuum line. Check for any nicks or bends in that line. If its a mechanical gauge running all the way to the cabin, be mindful of pinches to the line as well.
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Old 06-19-18, 05:56 AM
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My car also tops out at 5 psi, tries to nudge to 6 psi at 4500 rpm. Idle vacuum is about 12 in Hg.
it could mean any number of things unfortunately. But air is leaking from the system somewhere between the turbo outlet and rotor housing. That's what makes this such a pain. Additionally, my car pops and backfires when coming off the throttle but in gear, not bad, but it seems unrefined.

For reference, pressure should be around 8-10 psi and vacuum at idle should be more negative than 15 in Hg.

Gotta start peeling back the layers of this stinky onion. One thing I'm not looko g forward to is the additional h3at that will come with more power. My engine is already thermonuclear hot after driving it under low load 1 mile to the Autozone, even if I let it sit with the hood open...

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Old 06-19-18, 08:33 AM
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Originally Posted by j a r o d View Post
My engine is already thermonuclear hot after driving it under low load 1 mile to the Autozone, even if I let it sit with the hood open...
That's not good - do you still have the pre-cat in the downpipe? Could be clogged therefore causing your heat issues.
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Old 06-19-18, 08:40 AM
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Originally Posted by 0Cylinders View Post
Hey Everyone,

I just want to say thanks to anyone in advance that takes the time to respond to me here. I know I can find what I am looking for but I would like to get this figured out sooner than later. I just posted in the new members section as I am just returning after being gone for like almost 10 years. I am looking at a low mileage Silver FD touring 93 model that looks impeccable. I took a brief ride with the owner last Saturday before he had to leave town. The car sounded and felt great, idled great, hardly any smoke after sitting for a couple of weeks on a trickle charger and without any metering all would seem well. Towards the end after several attempts to push the turbos I noticed the aftermarket boost gauge was reading only about 5 psi consistently through gears even mashing it on the highway, and at idle I believe it was sitting in the negative (cant recall if this is normal or it should be at 0).

1) Could there be a vacuum hose leak causing the boost to be weak? I seem to remember this being a possible problem with my turbos in my 93 touring back in the day and it was the first thing I totally replaced on the car.
2) I forget what connections are made for the boost gauge, could it be improperly connected and showing less boost than there is?
3) What is the easiest way for me to verify the boost pressure? I do not have any of my old parts like Greddy boost controller or Apexi Power FC or anything.
4) Anyone in Colorado reading this want to check her out with me? =D

Vacuum at idle will be negative. You can eliminate the gauge as an issue by taking your own working mechanical gauge, a long length of tube and hooking it to a spare nipple on the UIM out the hood and in through the passenger window. Then you can watch the boost pattern yourself.

If it is indeed maxing out at 5 PSI there are several possible causes:
  1. Torn/split intake-turbo-intercooler-throttle body pipe connectors
  2. cracked crossover pipe (if plastic)
  3. cracked vacuum chamber
  4. split/disconnected vacuum hoses
  5. faulty solenoids
You can look for these things (#1 is pretty obvious) or do a smoke test, but in my opinion with these cars if you can, the best bet is to get a reputable shop to do a pre purchase inspection for you as that way you can get compression numbers as well. Then you really know what you are buying.

Good Luck

Last edited by FEED AFFLUX v5; 06-19-18 at 08:51 AM.
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Old 06-19-18, 12:03 PM
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My car also tops out at 5 psi, tries to nudge to 6 psi at 4500 rpm. Idle vacuum is about 12 in Hg.

My engine is already thermonuclear hot after driving it under low load 1 mile to the Autozone, even if I let it sit with the hood open...
Low boost and very fast warm-up or running hot are signs of a clogged cat as noted above.

Very fast warm-up or running hot and normal or overboosting are signs of a miss-fire or retarded ignition (sending a bunch of air& fuel into the catalytic).

The second condition over time will cause the first condition from overheating the catalytic.
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Old 06-19-18, 01:18 PM
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Thank you gents, downpipe is being replaced in short order. Hope my symptoms and advice received can help the OP in some way.
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Old 06-20-18, 09:01 AM
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Thank you so much!

Today is the day! I am going to pick her up!!!!!! I will examine the engine bay again look for any kinks or cracked hoses. I think I recall this fellow saying he had vacuum hoses redone fairly recently. I am going to get it either way and have it checked out in August at 5250 Performance in Longmont, Colorado for a compression test and vacuum leaks. I just wanted to reaffirm my initial thinking and get some answers from gurus like yourselves that maybe have not taken a break like I have from the rotary scene.
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Old 06-20-18, 09:03 AM
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Originally Posted by 0Cylinders View Post
Thank you so much!

Today is the day! I am going to pick her up!!!!!! I will examine the engine bay again look for any kinks or cracked hoses. I think I recall this fellow saying he had vacuum hoses redone fairly recently. I am going to get it either way and have it checked out in August at 5250 Performance in Longmont, Colorado for a compression test and vacuum leaks. I just wanted to reaffirm my initial thinking and get some answers from gurus like yourselves that maybe have not taken a break like I have from the rotary scene.
If it has never boosted properly since he did that it might be as simple as a disconnected hose or missing cap.

Have fun picking up your new toy, we all have to make sure to enjoy the good things in life when they happen. Keep us all posted on your progress
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Old 06-20-18, 11:42 AM
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If he had the vacuum hoses done recently and then it started boosting 5psi it would most likely be because the stock restrictor pills were not placed in the new short wastegate and turbo pre-control actuator hoses right down by the actuators.
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Old 06-22-18, 03:07 PM
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Originally Posted by FEED AFFLUX v5 View Post
Vacuum at idle will be negative. You can eliminate the gauge as an issue by taking your own working mechanical gauge, a long length of tube and hooking it to a spare nipple on the UIM out the hood and in through the passenger window. Then you can watch the boost pattern yourself.

If it is indeed maxing out at 5 PSI there are several possible causes:
  1. Torn/split intake-turbo-intercooler-throttle body pipe connectors
  2. cracked crossover pipe (if plastic)
  3. cracked vacuum chamber
  4. split/disconnected vacuum hoses
  5. faulty solenoids
You can look for these things (#1 is pretty obvious) or do a smoke test, but in my opinion with these cars if you can, the best bet is to get a reputable shop to do a pre purchase inspection for you as that way you can get compression numbers as well. Then you really know what you are buying.

Good Luck
Thank you for the further advice. Of course I would have to go pick this thing up during a busy school week. Driving it home vacuum was sitting at like -10 and max psi was hitting like 4-5. It does not feel like the secondary turbo is kicking in at all and it sound like there is a significant amount of air leaking somewhere during acceleration. I picked up a basic Bosch boost gauge and will try to connect it and do some further diagnosis. I forgot to bring it with me when I went to pick it up and make the final transaction! I have an appointment for a compression test and vacuum leak check August 2nd, but will continue to poke around at it until I make it in. Owner said the car has always driven this way and since he installed the boost gauge that is how it always read so he assumed it normal...

I am also now finding out that the passenger door speaker does not work and the acoustiwave bass unit sounds blown... But she does have some low mileage and I am still happy with the price given the climate of the market. Thank you thank you thank you. It feels good to be back!
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Old 06-24-18, 10:59 AM
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Thanks again for the help!

I picked it up on Wednesday right in the middle of a busy school week. I managed to follow a silver RX-8 most of the way home too right when I got on the highway. I felt like it was meant to be. I pulled in front of the girl driving it for a second and gave her a thumbs up. When she passed me she had a very distinct "wtf do you want man?" face. I don't think she got it.

I have only driven it a few times thus far and definitely some weirdness....
  • Vacuum at idle sits <10
  • Max psi is about 5
  • Secondary turbo is not engaging at all it seems. There is no secondary pull feeling and it sounds like someone just deflated a large balloon while accelerating.
  • Also unrelated but somewhat aggravating is I was not told the Bose system sounded like it is underwater before buying it. I was so concerned looking over the car I did not even listen to it. Being an audio guy I am especially disappointed in my self. Now the debate to try to fix the bose or put in a bad *** Focal begins.
Over all it is frigging beautiful and I am thankful for the low miles. I still have that appointment August 2nd for a compression test and vacuum leak check. Today I plan to pop the hood and check the bullet points you listed such as the throttle body and crossover pieces. The vacuum lines do look fairly healthy on the surface.

You rock!!!!!!



-Jake
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Old 06-28-18, 06:44 PM
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Trouble shooting update

So I took some time to look around at the connections the other day and I thought I found the problem. The bottom of the crossover pipe was not fastened all the way down and was loose at the collar. I pushed it down and tightened it up and thought I was going to fix my problem. So after one more quick once over on what vacuum lines I could see I took it out for a spin. The leaky air sound quieted down a bit but it sound like there still may be some air leaking some where. Boost patterns are still the same sitting at -8 hgr and hitting about 5 psi max. The secondary turbo is definitely kicking in though, although it seems really around 5K that it begins to spool up. Boost remains 5 psi throughout but there are two pulls now and a slightly less exaggerated leak sound.

I guess the next thing to do is take out the intercooler pipes and crossover pipes and inspect for cracks? I may just take the easy way out and order a Pettit charge cooler until I decide what kind of crazy build I want to do to this thing in the next few years.

The vacuum lines do look good over all there are a few that do not have any sort of zip tie or fastener around a couple solenoids but they feel tight.

There are two bolts holding the crossover pipe in place which I am missing one of. This may have caused the crossover pipe to loosen up and slide up the y-pipe.
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Old 07-17-18, 10:28 PM
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Any further progress on this?

Have you checked for the ECU throwing any codes (possibly putting you into limp mode)?
Did you check the restrictor pills as per BLUE TII comment?

If you aren't having any luck at least your mechanical appointment is coming up soon, but hopefully we can help
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Old 07-18-18, 01:33 AM
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Some updates thanks for asking!

I think I mentioned in a previous comment that the stock crossover pipe was loose and coming off the y-pipe. I tightened it down and the massive boost leak sound went away and both turbos are kicking in. The boost gauge is reading incorrectly is all I think it is now. There are like 4 hoses of varying sizes that are not tightened down enough and leaking air between each other. I have a new vacuum line that I could replace the whole run with but I got nervous attempting to pull the A pillar off the other day. I have also been pinching pennies and avoiding a cheap mechanical boost gauge to verify because today is my birthday and I just put nice little order through for some new Volks to replace the 5Zigens on there now. Sounds like I will be the only person in the US with these rims too. Very excited.

Sorry tangent! I have an appointment on August 2nd at one of the only shops that advertises they work on rotaries for a compression test and vacuum check. I have been debating on just getting a compression tester and doing it my self but I feel this could be a good ice breaker job for this shop and get to know them better. As much as I want to do everything on this car myself I am currently garageless and winter will be here before we know it.

Volk G27PM LTD Pressed Ash Gray 18x9 38

Last edited by 0Cylinders; 07-18-18 at 01:34 AM. Reason: Clarification
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