Sidewayz240'

hey guys so my redtop SR wont catch a falling rev but idles perfectly fine for example, if I'm driving down the road tacking 3k rpms and I knock it into neutral the motor will go down below 500 rpms and die out. this happens with any falling rev, even a stationary rev up to 2k , it will die on its way back down. ive done some reading couldn't find many threads but the ones I did were talking about recirculating your bov. mine has always been recirculated although it is on the cold side. could that be an issue ? also its a stock redtop with a turbo mani, fmic, no cat and an apexi cat back. any input would be greatly appreciated ! thanks guys.

Kingtal0n

set the minimum airflow of the engine using the adjustment on the IACV (a plastic screw you turn with a screwdriver) so that without any help from the IACV your minimum idle is just below the desired idle.

For example if you want a stock 850rpm idle, you would adjust the airflow minimum for an 800rpm idle. This way 800rpm is the lowest the engine will ever go, then the computer control can add the next 50rpm of extra air.

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Sidewayz240'

Hey king thabks for the reply. I tried adjusting the iacv i upped the idle from 800 to about 9-950 but it still did the same thing. I even adjusted the throttle body screw thinking if the throttle body is slightly open it wouldnt die out but it still dies out when you drop the rev from 2k+. It sits at idle perfectly , im stumped..

Kingtal0n

leaking plumbing (atmospheric bypass) / boost leak will cause the engine to go rich and stumble, common symptom. Whats the wideband say when you lift from the throttle and the engine tries to die? Confirm with wideband going 10:1 flat rich at that moment that it is, indeed, a leak in the plumbing.

IF you dont have a wideband thats why you never caught this issue. DO a pressure test (fill plumbing with 15psi~ and find leaks) followed by BLOCKING the bypass so it cannot operate to confirm.

Also make sure when you set the base airflow that the IACV is fully closed electronically (disconnected at max closed position). You can do this the hard way by blocking the flow to the IACV via the auxiliary tube, using a vice grip or simply plug it with a marker.

Sidewayz240'

How can i get 15 psi to pressure test it ?
When setting iacv you said it should be fully closed and disconnected . Do i do that with the motor off or how does that work ?

Sidewayz240'

Thanks again king . Appreciate the help

Kingtal0n

use an air compressor and search how to do a "boost leak test". The quickest way is to make an PVC endcap from home depot with a tire valve stem in it, then replace the air filter with it. Make sure you disconnect the crankcase feed tube that runs from the pre-compressor inlet to the valve cover on the sr20det engines. You do not want to pressurize the crank case, only the turbo plumbing. I use a marker to block this tube. If you are un-sure or new to crankcase / PCV setups, the simple way to make sure is pull the dip stick out and feel for pressure there. If you detect any pressure in the dip stick tube while pushing compressed air into the plumbing you should STOP IMMEDIATELY and figure out how it is getting in there. The intake side has a check valve (pcv valve) which stops the "boost pressure" from going into the crankcase so you can leave that tube connected to the valve cover.



IACV<<<
sr20 idle control valves are notorious for acting like 20 year old crusty electronics, that is, they misbehave frequently. Instead of fooling too much with the tube/wiring I would simply raise the idle bit by bit using a screwdriver until the engine seems like it is idling happy enough (around 850rpm for stock engines). The base airflow is adjusted by a little plastic bolt in the iacv valve, you probably have seen it already, and that is the only place you should be using to raise the idle. You can start by raising the idle too high at first, then drive the car hot, fully warmed up, and bit by bit lower the idle until satisfactory.

The stumble you note is likely due to the over-fueling of the engine (watch the wideband) as the majority of idle airflow tends to flow through the IACV bypass (adjusted by the plastic bolt) and not the ECU's commands (Which seem to do little when the valves become so old and full of carbon). Another issue is the coolant flowing through the throttle body- make sure it has these lines connected. The idle is affected by a wax element which expands when the engine gets warm coolant flowing through the throttle body, lowering the idle speed.

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5.3/4l80e/turbo 240sx playthrough:
https://tinyurl.com/turbo240

Sidewayz240'

Hey king thank you for all the help man. Greatly appreciated ! You were right , i had a leak in the plumbing. Took bov off plugged it up and the symptoms were gone. Looks like i need a new bov now. Can i rebuild the bov ?

Kingtal0n

what kind of bypass, what kind of turbo, how many miles on both, the more i know about the engine/setup and mileage the more I can weight the pros and cons of fixing/rebuilding this and that.

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5.3/4l80e/turbo 240sx playthrough:
https://tinyurl.com/turbo240

Sidewayz240'

Blitz bov. The whole motor is stock besides Fmic and turbo mani/downpipe. The turbo is a stock t25 unknown miles but when my old one gave out i replaced it with this one and it was in good condition. And on the motor it has about 76k

Kingtal0n

Im not sure about that bypass you need to contact the manufacturer. A stock bypass works great if you recirculate and use 11~psi of boost (about as much as a T-25 should see)

Greddy type-S is my favorite for T-25 turbos, recirculated. but the rebuild kit for those is pricey.

I would avoid hks SSQV style. A push type recirculated is ideal for maf car with t-25/t28

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5.3/4l80e/turbo 240sx playthrough:
https://tinyurl.com/turbo240

steve shadows

The best way to check for boost leaks is to use a smoke/vapor tester so you can spot even hairline leaks. Always the best way to go if you can rent one for example or simulate vapor into the intake and vacuum system..

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Kingtal0n

I disagree. Some leaks wont show up until 2psi+ so you need the pressure to put force on couplers and tubes. Not to mention you need the 15psi or 20psi of pressure to ensure everything stays put once boost is actually achieved. A smoke test shows good and then you boost the car and something opens up, pops off, or comes loose is common. And then the owner is scratching their head "but it tested leak free??" comes on the forums and types " boost leak tested.... still having issue..." without mentioning they never applied actual boost pressure during the test.

turrible just turrible

__________________
5.3/4l80e/turbo 240sx playthrough:
https://tinyurl.com/turbo240