So basically now it won't pressurize above 0 psi or so.. it seems like it MIGHT be reading a little above 0 psi, but definitely doesn't go even 1/4 way up the boost portion of the gauge (450mmHG is full I think, it looks like it's reading about 10mmHG MAYBE at the most near redline)

 

Could be blocked CATS, have you replaced the stock ones yet?
Could be vacuum leaks. Could be... aww crap, it could be a billion little things. Such is life with a TII.

 

I think the cats are hollowed.. the sucker failed emissions bad (like more than double the allowable limits in Ohio, which is fairly easygoing with emissions).

A vacuum leak would cause it to boost that low? I just took it out again and it seems like it goes a little bit over 0 psi, but not really close to even 1/4 the way past 0.

I can't tell if it's just not boosting past that, or if the gauge isn't displaying correct, or maybe the sensor isn't reading correct? One of those 3, I dunno

 

are you looking at the stock boost gauge?

 

I know I had problems with the pre-cat being blocked, and I would barely get any boost. Cleared it out, got full boost.
Do you hear a 'whooshing' sound when you accelerate? (could be a few hoses cracked or disconnected.
Ummm, I am assuming you aren't getting your usual acceleration, as the stock boost gauge is pathetic at best. You can still have boost, and not see it on the gauge.

 

I would definately get the cats checked, as this would also cause you to fail emissions.

 

well I can sneak by emissions with a repair waiver cap (if I show that I spent $300 on "emission related equipment" ).

I REALLY don't want to try to unclog the precat, I'm so sick of working on this thing.. I'm really just wondering if it's just not reading correctly. Could it be that the pressure sensor reads the vacuum fine, but not boost?

 

oh yeah.. and if the stock boost gauge is crappy, then is that the result of the pressure sensor being crappy? Becuase if so, then won't that cause the fuel to be insufficient for the boost I'm running?

If I hook up a pressure sensor and read like 7 or 8 (or whatever) psi but the stock boost gauge is only showing like 1 psi (MAYBE 1 psi, maybe not even that much) then I'm going to have to assume that the fuel being delivered is only for 1 psi

 

Well, I just had a big issue with my boost sensor wiring. Seems like you understand that the stock boost gauge is dependant on the sensor. A lot of people say don't pay attention to it, but resolving this issue fixed my car. The old wiring had been so chewed up by previous owners I just ran a new signal wire from the sensor to the ECU, then spliced the stock boost gauge back into the new wire. My car runs perfect now... but the stock boost gauge isn't very accurate. At full boost (8ish PSI) I'm about halfway up the stock gauge.

Do you have a real boost gauge? That would tell what is actually happening, instead of what the sensor thinks is happening. I would check your wiring though if the stock boost gauge isn't moving too much. it's worth a shot.

 

hey.. you're in Novi.. mind if we looked things over on my car comparing it to yorus?

By the way, I hooked up a testing pressure gauge and it only boosts to 5.5 psi or so, while the boost gauge showed only like 1 psi or something.. stupid electronics.

One more thing, I was told that the FD's don't add fuel or anything to compensate for the boost pressure, everything is done by the Airflow meters, not by boost?

 

I think you mean FCs, but yes. The stock FC ECU does not change the fuel maps based on the pressure sensor input - only the AFM. The pressure sensor is only used to retard timing - it's only slightly more complex than the MX-6 turbo engine control, which has a boost switch(!). RETed tested this, and I've verified it with an o-scope (at constant RPM, duty cycle does not change with varying pressure sensor output).

Basically you need to verify the stock boost gauge reading with a "real" gauge. Debug from there.

Brandon

 

no.. my friend owns an FD, and if it's not on an FD, it's probably not on an FC either.. that was my thinking.

So is it a serious problem if it's not retarding timing correctly? If so how bad? Do you think it's a gauge problem, or a pressure sensor problem? Have the pressure sensors been known to go bad?

 

The FD uses a MAP-based system, while the FC uses an MAF-based system. Apples and oranges.

The timing issue is an engine-control-related problem, so you should get it fixed. Period. Pressure sensors go bad, but you won't know if it's a sensor problem or a low boost problem until you actually do some debugging. So go get a real boost gauge.

Brandon

 

I hate to dispute anyone but the boost pressure sensor does have an effect on the amount of fuel that is injected. If you don't believe that, disconnect that sensor and see how little power you have. I just fixed a similar problem on my '87 TII. I had done all the mechanical stuff to the car. Turns out the gauge was bad....it's pretty rare for the boost pressure sensor to fail. I took out the cluster and cleaned the contacts on the circuit board that the gauge is mounted to. Tightend all the screws and now it boosts all the way to the top.

Todd

 

Barwick, you should not assume anything. The pressure sensor is probably sending the correct signal to the ECU and the gauge. The gauge is just not reading correctly. Another symptom of this is that with the key in the on position but the engine off, the gauge does not read all the way up to 0. It should be squarely on the mark for 0 (on an '87 TII anyway).

Todd

 

yeah that's what I thought, turned to on, mechanical gauge reads 0, stock gauge reads like -20. Start the engine, mechanical reads like -15 or -20 or something I forget, the stock one reads like well below -20 (pegged down). Pull the vacuum line off, and it goes back to -20 stock, (should read 0 when there's no vacuum).. blow on it really hard, still hardly reaches 0.

You said you cleaned out the connectors and it worked ok now?

 

I would think that you just proved that your stock gauge is not working, and that you are getting normal readings from the mechanical gauge. With the mechanical gauge in place, what kind of readings are you getting when driving under acceleration?

 

5.5 psi on the mech gauge, about 1 or 2 psi on the stock one at that same time. But that's at a 1st and 2nd gear full run, not on the freeway in 3rd.

 

I know my 87TII has boost between 5.5 and 6.5, but I believe yours should have about 8 psi.
Is the boost constant, or does it fluctuate (ie, one time it's 6, one time it's 5 etc), if so, I still suspect blocked cats. If it's steady, maybe a small vacuum leak.

 

Think about what you've just said... are you talking about unplugging the connector or the vacuum source? If the first, then the ECU is in limp mode, and retards the hell out of the timing; thus, no power. If the vacuum source is off, the engine thinks it's seeing atmospheric - and it retards the timing again.

In any case, it's a non-issue that the ECU does not increase the injector duty cycle under boost. This has been independently verified by both myself and RETed on an S5 and S4, respectively. When pressure (from a MityVac) is applied to the pressure sensor at a constant RPM (i.e. no change in AFM reading), the injector duty cycle does not change.

The pressure sensor is simply a variable resistor. The boost gauge is essentially a voltmeter that reads from 0VDC at vacuum to ~4VDC at its max reading (14.7 PSIG). Your contacts being cruddy increased the resistance of the pressure sensor, hence, it read low.

But yes, Barwick has just isolated the pressure sensor as the source of his problem. Test it per the FSM and replace if needed. Don't worry too much about driving it around - just try to stay out of boost.

Brandon

 

I've talked to a few people and they said it's probably not the pressure sensor, but rather the boost gauge that's bad, becuase of the symptoms I've got with it.

I'll test the pressure sensor though, there's a way to do it in the FSM? I've got a Haynes I think, I'll try.

 

The question you have to ask yourself is this... do you still have normal acceleration, if so, then just replace the gauge with the mechanical one, if you're not, then you have other problems. Either way, the gauge will not affect what kind of boost you're getting.

 

I don't know, I've never driven the car besides after the engine swap, and this bug started with the engine swap

 

Hey Barwick, how far is Euclid from Novi? I actually am on summer break, and can't work because I developed tendonitis at my summer job. If you want to hop up here I can help ya, but I don't know too much

 

Actually I'm in Royal Oak right now for work, and going to Flint for school, so not that far at the moment.

But actually I just went down to Mitch's Auto and the guy looked at it, just a few things to work on.. including a nipple on the ACV is screwed up, but not too bad