S5 Low Boost…Sometimes
 

The Car:
1990 Turbo II
Completely stock setup
220k+ mi. on chassis
~100k mi. on 2nd engine

The Problem:
The car only achieves normal boost levels occasionally, more likely when it’s cold outside, but still not guaranteed. Sometimes even on hot days it will boost high up the stock boost gauge.

Now before you tell me that the stock boost gauge is garbage, I realize that. However, the stock gauge is a good indication of whether or not my car is running decent boost levels (and resulting in more power) or not. Every time the car has run very strong, the gauge deflects high up the scale. I have searched through many “stock boost gauge” threads, and supposedly the gauge is supposed to point to 0 (or close to it) when the ignition is ON, but car is off. My gauge has never reached 0, and rarely reaches above the –20. Most of the time, it only moves about 2 needle’s widths above the pegged position. Is this an indication of a bad pressure sensor? I haven’t tested it yet via the FSM.

Most of the time when I go for a drive I can’t get the gauge to read over 0 while giving the car hell. But every so often, it will get into the positive area of the gauge, sometimes deflecting about 3/4 of the way up. As said before, I understand that I do not know the exact boost being made at these levels, but the gauge reflects the fact that I can feel and hear the turbo making power or not. For this reason, I doubt that this is a cold solder joint problem with the gauge.

I know that my cats are probably toast and killing part of my boost, as I believe that they are the original ones. If that is the case, how does that explain very low boost sometimes, and normal boost at other times?

I’ve sprayed carb cleaner around the vacuum lines looking for vacuum leaks, but haven’t found any.

I recently broke off the AWS nipple that connects to the intercooler, and replaced the AWS unit. I had been getting a bucking at ~1500rpm if I was not on the gas. I just adjusted the TPS on Monday, and the hesitation is gone. The idle is a little low. It idles around 550-700 when first started. I have to give it a little gas for no more than a minute to wait for it to idle on it’s own. Once it warms up completely, the idle is about 700. When I turn the AC on, the idle gets bumped up to about 800 and is solid. So to me, that indicates that my BAC is working correctly.

The vehicle has no problems starting either cold or warm. It starts within the first 2 seconds of cranking either time.

I have neither checked the operation of the wastegate actuator nor the condition of the TID.

Thanks for any input.

- Jared

I just checked the voltage of the pressure sensor and am getting about 2.2 volts.

Went for a drive the other night and enjoyed a rock solid idle at about 800 until it was fully warmed up. However, I did feel a very slight bucking when it got below 1500 rpm when slowing down in gear. Then the idle lowered and bounced ever-so-slightly around 500-700 rpm, but the bucking was gone. Methinks that the TPS was actually not adjusted perfectly.

Enough with that. By the time I got on the interstate, I gave the car some hell, and the boost gauge defected about 3/4 up the gauge (see previous post) and felt the power again. Drove about 10 miles up and turned around. Only sat still in traffic for < 2 minutes. On the way back, the gauge would read no higher than 0 and power wasn't quite there. By the way, these are in 4th and 5th gear. I understand lower boost in lower gears.

Took a look at the cats at idle after driving the car hard the last 5 miles before shutting it off. They were not glowing (checking for signs of being clogged). Still pretty sure they are the stockers, and by 220k miles they have to be clogged. Exhaust probably isn't doing so well either.

I'm looking into replacements for both but am hesitant on the new tones coming from the car. I do NOT want something ricey, tinny, or anything just plain "ghey" on the car. I like to keep it as stock as possible. (The new AWS solenoid is bugging me because its brand new and most everything else under the hood shows some age.)

Also, do you think it's possible that my secondary throttle plates aren't opening fully at times? Could this help explain the coming and going in boost/power? I might try to wire them open for kicks. Also, a good cleaning with carb cleaner probably wouldn't hurt.

Still haven't found any vacuum leaks of any sort.

Still thinking about cat/muffler replacement with the RB system. I just don't want to go off and replace things without making absolutely sure that that is the problem.

Today, Imma check the wastegate actuator to see if it's stuck open or something obstructing it from closing all the way. What's the easiest way of getting to it? Removing the air box, AFM, etc? Still not sure if this can explain my coming-and-going (but mostly going) boost.

Use a pressure tester to check for boost leaks from bad gaskets, vac lines, etc. www.boostpro.net/prodtester.html sells a universal tester that will work. You can also make your own out of easily available hardware store parts

Are you sure the stock boost controller is operating correctly? I tried two on my first S5 TurboII and neither worked, I think it was an ECU actuation problem. I just ran at 5.5 psi (measured on a mechanical boost gauge) and it was plenty fast for my purposes.

Other than that, the wastegate could be stuck, cats clogged, something, lots of other things it may possibly maybe be.

May be a silly question, but will this check ALL my vacuum lines and not just my TID, IC piping? I plan on hooking this thing up at the TID and pressurizing to ~ 7psi. Hopefully it doesn't leak thru the exhaust too fast before I can hear anything leaking.

Pressure Test Results
 

Today I used that pressure guage that arghx described. I removed the stock airbox and AFM and placed the gauge inside the TID. I make certain that it was very tight.

I could not pressurize the system at all, not even 1 PSI!. The guage didn't budge. When I let off of the air compressor, it sounded like air was escaping near the turbo shield. Is my wastegate stuck open? Bad gasket? Am I even doing this right?

Please help. I'm so sick of having a boostless turbo RX-7. :(

Is there something obvious that I'm doing wrong with trying to pressurize the intake, or is the leak really that bad?

Drove the car for the first time since early August.

For those who stumble across this thread in the future, my turbo was shot. I'm boosting up to about the +45 on the factory gauge now.

I suppose I'm happy...