3GENJDM: 3rd Gen low boost trouble shoot

[e30]

for some of you that might have read my other thread, my 3rd gen isn't running optimally. It is still running pig rich and not pulling hard to redline.

I just installed a STRI electronic boost gauge and its telling me i'm boosting about 7PSI. I know the most common reason to why it might be in a low boost mode is: 1) the o2 sensor is bad 2)the speedo is disconnected.

I've recently replaced the o2 sensor with a brand new OEM one thinking it might solve the rich fuel mixture problem, but it only improved mileage slightly. My speedo is also currently hooked up and working but the cruise control wires aren't hooked up.

My car is also mildly modded with berk 3" DP and dual exhaust. The car also has some kind of ****ty intake on it but i guess it does the job. Besides these mods, the engine is completely stock. As far as i know, the car is also running GEN III ecu, igniter and coil.

Does anybody know what else could put the car into a low boost mode? I'm hoping that by solving this, my car won't run so rich afterwards as well. It backfires while cruising!

[e30]

with an aftermarket intake, where do i plug in my air intake temp sensor? does anybody have a pic of their set up?

[PETC]

If there isn't an input for the air intake temp sensor on your intake you can simply mount it somewhere near the intake, it's really not that crucial.

As for your low boost problem, it sounds like you're running in limp mode. My gen 3 did the same thing after I got it installed because the speedo wasn't hooked up properly (Like you mentioned)

The speedo worked fine, but the some wire for the ECU wasn't grounded/hooked up properly. I suggest checking that.

[e30]

does somebody have a diagram of how its actually supposed ot be hooked up? when i was installing it, i've read that if you dont' need cruise control, i beleive the purple wire doesn't need to be hooked up. its been a while since i've put it in so i think i need a refresher.

[e30]

Another wierd thing i noticed with the car was that it barely lets me rev past 6k. I have the gen III igniter and coil installed.

[SuperNutz]

boost leak or map sensor, speedo have nothing to do with limp mode.

[PETC]

Yes it does.

[rocklizzard91]

i must revive this thread.
i didnt know that the gen3 reved past 6k. i just got mine in at it revs to 6k and im having boost issues too

how does the speedo need to be hooked up?
its origionally a 91 NA
so its a cable speedo...
any idea what i need to do to hook it up?
the plugs dont match

[stewart]

The plug on a 91na is for cruse. Use 91 92 turbo gear that goes in trans , hook up cable. sensor for ecu is on mech speedo in dash. speedo operates elect. power stearing motor, cuts it off and starts it at diff speeds

[rocklizzard91]

oh ok.

why get the turbo gear then?
is it different than the NA gear?

why might i only be reving to 6k then?
what should it rev to?

[stewart]

About 7300 rpm.I just bought new turbo gear assy , about 210.00 at toyota, search here or mr2oc and youll find part #s some of your other parts will work. the wires that were for elec. speedo on 3rd gen harness hook up to cruse controle but you have to cut plug and hard wire it. check map sensor make sure you have the right one

[rocklizzard91]

im not sure what youre saying exactly....

which map sensor should i have?

[stewart]

Its smaller than 2nd gen map. The sticker on map is purple and 2nd gen is green Search p# to be exact.

[rocklizzard91]

i may have the wrong map then...i think mines green

[rcracin]

wtf all this time you had the wrong map sensor? it HAS to be purple dude. I'm sure you can have that taken care of with Baktasht.

[CJMR2T]

89420-17050 is the number for the correct MAP for a Gen 3.... DO NOT go by color. Color is not specific. I've seen green and purple ones with the correct numbers. For that matter, mine is silver (probably carded).

While on the subject, MAP will not cause a Rev limit. However, it will cause fuel cut at a much lower boost level since it'll hit the specified voltage to trigger fuel cut. This however would not specifically happen at the same RPM every time.

You could very well have a false reading tach. I have that exact issue. Mine reads fine at idle but progressively becomes off as rpm increases. At 7000rpm (true rpm verified with my Fluke 88 multi-meter), my tach reads roughly 5800rpm. I also datalogged the NE signal from te distributor just to make sure that real engine rpm matched what my Fluke was saying based on ignitor output signal.

Back to low boost, could be a number of things. The SPD signal input is a bit of a questionable situation. For some reason, not all seem to go into low boost mode with lack of SPD signal but most do.... also, temp signal is important for factory boost control.

The big question that needs to be asked is if boost is just low but rises smoothly or if its low and erratic in its rise...

One trick to find out if the ecu is staying in low boost mod is to hook up a length of wire to the TVSV control wire (don't recall which wire off hand), and run it into the cabin, hook it to a small light bulb or standard 12v test light and the other end to a 12v source. If the light turns on once the engine is up to operation temp, then the ecu is allowing high boost. If the light never turns on then its staying in low boost.
Summary:
Light on = high boost
Light off = low boost

I actually inverted the setup with a relay so that light on is low boost and off is high boost. I have a small LED in the dash and it therefore works as sorta a knock light. If something goes wrong that the ecu wants to run the engine in low boost (such as knock), it turns on the light.

[rcracin]

Hmm never seen a gen3 map sensor that wasn't purple. All the st205 and mr2 ones are purple. Now I know for future purposes. Thanks cjmr2t

What did you mean by carded?

Also, the st205 gen3 map sensor carries a different part number but works just the same does it not?

[CJMR2T]

Sorry, I'm posting from my phone and that was supposed to say faded but for some reason the phone auto corrected to carded and I didn't notice.

The color thing was a year change thing that Denso did. The few green ones that had the same correct number were 94/95 engines. I'm not positive but I suspect that denso changed the color with 96+. Mines a 96 but its silver (again, I suspect its faded).

The color thing can be a good judge most of the time but its not a guarantee. There's a similar issue with many 89 model year Toyota's with the ecu based on color of the connectors. Sumitomo who at the time was making all the connectors and wiring decided to change the material and color of the connectors and added a few new connectors to its lineup. Denso started using the new connectors in some of the ecu's. So in 89 it became a problem, you possibly had yellow or gray and new or old connectors. This a problem with supra's cause many people think that you either need a yellow connector ecu or a gray connector one but in (yellow being early style and gray being late). In 89 though its possible to have gray harness connectors but they are the old style so you need to find an early ecu (with yellow connector).

This is why you have to watch with going by color and should always go by part number.

Yes, the st205 uses the same sensor but has a different number. The number is different cause the mounting bracket is different and Toyota only supplies it with the bracket thus has to have a different number....

[rocklizzard91]

actually mines orange..... ill have to check the part number :P

[rcracin]

Orange is usually stock gen2.