Boost gauge sticks then jumps with engine response

[Ronquest]

Hi, I just purchased a 91 turbo about 3 weeks ago. Engine and turbo rebuilt 24K ago with no mods. I've done an ignition tune-up with Toyota parts and replaced a cracked turbo inlet boot. Also added a K&N cone filter (noisy).

The problem is the boost gauge needle will stick and then jump up to position (about half way, where it should be) and the engine performance will respond slightly at the same moment as if something had just been corrected. This happens mostly when taking off and driving normally but not every single time. It seem as if it needs to be at a certain pressure to become unstuck (maybe around 6 lbs. or so and 3-4k rpm at 1/3 throttle, sometimes this varies a little) While cruising if I keep the gauge from resting and get back on the gas it responds properly with the sound of the turbo. The turbo gauge does go to full boost. When reving out at WOT the engine does not seem to hessitate or miss. To give my best guess I would say it's like the engine is running lean while the turbo gauge is stuck and when the needle moves it richens up, not a huge difference but definitely noticeable. The turbo sounds unaffected by this. Is there something mechanical that could be sticking or a bad sensor?
What Controls the gauge?

ps. the problem existed before the K&N filter and tune-up

Thanks for any help and look forward to getting to know you guys.

[Piddy]

Is this a stock boost gauge you are talking about or an aftermarket one? It kinda sounds like ignition but you said you had replaced 'everything'? What does that include? Just the plugs?... or did you do the wires too?... or the entire dizzy rotor and cap?

If it is sticking at around 6-7lbs the car is going into 'safe mode' for one reason or another. The only time I have personally experienced that is when my plug wires were old.

[Strat42586]

Mine's stock as well, except for the Toyota ignition tune up and the Apexi power intake. And my stock boost gauge does the same thing. Stock boost gauge is slow to respond and seems extremely inaccurate.

[Piddy]

What was all replaced in the 'Toyota ignition tune up'?

[Strat42586]

Mine included distributor cap, rotor, wires and NGK plugs. I would assume his was the same.

[Piddy]

AH!... I didn't notice it was two different people posting.

[Ronquest]

The tune-up was rotor, cap, wires and plugs gapped to .032". Stock boost level. The boost gauge is the stock one. Also this is beyond a slow gauge, it sticks only at the resting position and is accompanied by engine response. If I try, by staying light on the throttle but still in boost I can keep the gauge stuck for several gear changes, the engine also seems down on a few horsepower as if it may be lean until the gauge jumps.

What does the computer use to creat a boost signal for the stock gauge? I know (in other cars) most of these factory fake boost gauges formulate the AFM, rpm, throttle position ect.. to come up with a signal that moves the gauge for our amusement.

As for safe mode wouldn't it still move the gauge?

Thanks, Ron

[Piddy]

I have an NA swap so I never had a stock boost gauge... but I'm pretty sure the stock gauge is an electronic one. The stock gauge is basically worthless though... it doesn't tell any pressure readings and just kinda bounces back and forth for amusment like you said.

I'm assuming this isn't just turbo lag you are feeling? What happens... say in 3rd gear... if you just push the pedal down WOT? Does it build full boost or is the computer pulling it back to 6psi?

You should look into getting an aftermarket boost gauge... or maybe borrowing someones to see when it's acting up.

[MR2Jedi]

Like Ishcoleobo said, the stock boost gauge is worthless. Mine does the same thing and I'm not having any engine/turbo problems. Best thing is to buy and install EBC with a real time boost gauge and squeeze all the boost u can out of stock turbo.

[Ronquest]

I know what your saying about the gauge. My problem is not just with the gauge. Please read my post, I tried to be specific.

Thanks for your replies, Ron

[JekylandHyde]

Hey Ron, from what I can tell from reading your post your gauge functioning is an EFFECT and your are trying to locate the CAUSE. It sounds like something is sticking.

What BOV do you have (if any)?

I would highly recommend getting an aftermarekt gauge so yo ucan see more accurately what is going on. VDO makes a simple black face gauge that reads vac & pressure for only $26 (see summitraing.com).

At this point, my guesses are:
*blow-off valve sticking
*throttle plate sticking
*weird altering vac leak

Has it done this since you have owned the car? (always this way)
Can you get a photo of your engine bay?

[Ronquest]

JekylandHyde, it has the stock BOV (I pulled it off and tested it and it tested fine) I can hear it easily with the K&N cone filter setup and it sounds fine (doesn't make an audible change when the gauge jumps).

I am buying a VDO boost gauge (seem to have better reviews than some others).

Can the throttle plate stick and still function with the gas pedel ?

It has done this for the 3 weeks I've owned it.

I'll take an engine picture and repost shortly.

Do you know what directly signals the boost gauge?

Thanks for your help.

[Ronquest]

Here's the link for the engine bay, ya I know it needs cleaned, I haven't had time yet.

http://moto-on.com/mr2.html

[JekylandHyde]

The boost gauge is signalled by the OEM MPA sensor.
All this sensor does is operate the OEM boost gauge and control fuel cut.

Ok, the car looks mostly stock except for the cone air filter.
Did you install that or did the P.O. (previous owner)?
There is a mistake (unfortunately costly) that many owners frequently make when installing a cone filter. They undo the two litte screws to the BIG wire going to the AFM (Air Flow Meter) when installing a cone filter. That is not how that wire comes off. If you or the P.O. undid those screws it would have damaged the AFM and would cause some sporadic running problems. NOT sure this is your issue, just throwing it out there.

Two other things I noted in your engine bay photos.

1. The Idle control valve line that comes into the intake stream right after the AFM seems to be oily. (big hose about 1" diameter). That is a were unusual place to have oil. I would check that out.

2. The X-bar has 2 llittle holes drilled in it on the driver side. Do you know what was mounted there? I suspect it was a MBC (manual boost controller). If so, I would check the vac lines around the turbo and the wastegate actuator to make sure they have been put back correctly.

[jackcannibal]

***QUOTE*** Like Ishcoleobo said, the stock boost gauge is worthless. Mine does the same thing and I'm not having any engine/turbo problems. Best thing is to buy and install EBC with a real time boost gauge and squeeze all the boost u can out of stock turbo.
__________________
rcody

***QUOTE ****









I strongly discourage adding any more gadgets before you can get the engine to run correctly in stock form. I also hate EBC's. It's just one more electrical failure waiting to happen. The most accurate and reliable are MBC's. Also, they don't short out, but they do look kinda ghetto.




This whole problem stinks of an intercooler piping leak to me, but I cannot say for sure. There's not enough information as to the nature of your problem. I assume you're saying your engine falls on it's nose until higher rpm's. If so, what are some other things? Is there any black/blue smoke? Is it just flat out stalling or is it just weak and chugging (what us 2-stroke motorcycle guys call "four-stroking")? I see that at WOT it does just fine, according to your post. This doesn't sound like a lean condition before spooling. Usually, a rich condition will cause loss of power until turbo spool occurs, at which time the increased airflow will lean it out enough to run better, but still not perfectly.


It's hard to determine where to start without enough information. Have you done any preliminary testing yourself? Have you checked the CTS, MAFS, BPS? These are things to consider. Even the TPS could be giving you trouble. Try to gather as much info. as you can and let us know.