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Guest Message by DevFuse

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Fuel cutoff bypass for hot wire MPFI

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7 replies to this topic

#1 All_talk


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Posted 05 April 2004 - 12:23 PM

I was reading through the ’89 FSM (is it normal to use a FSM for pleasure reading?:D) and found this bit of info (see pic). Looks like the fuel cut is suppose to come in at 9psi and if we determine the switches normal position (closed or open) we should be able to bypass it and defeat the cutoff. Or maybe we could find a way to adjust/replace it to work at a higher pressure. My RX is not ready for extra boost so I cant test it yet but looks promising. Maybe use it for fuel enrichment or water injection.

Another note, the oxygen sensor is not used under boost and the computer is running on a preset program, I wonder if this fact could be exploited and modify the pulse width of the injectors to enrich the mixture.

Just some thoughts


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#2 calebz


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Posted 05 April 2004 - 12:35 PM

Skip's been working on something for a while..

However, i run mine at 11-12 psi and have yet to have any problem with fuel cut..

I just got my intercooler last night, so boost may go a little higher.

#3 Subarutex


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Posted 05 April 2004 - 02:29 PM

Awesome! Now... which sensor is it specifically?

On my previous build I hit fuel cut a lot, I have yet to hit it on this build (still relatively low boost though).

It has previously been thought that the fuel cut was a function of the voltage coming from the MAF. I run an Apexi SAFC, which modifies the MAF signal to give me some control over enriching or leaning the engine. So, i was under the impression that i was modifying the MAF signal too much, which was putting it past the fuel cut voltage mark more often than other people. To remedy this, I have bought an HKS fuel cut defencer, it has yet to be installed. The basic premise behind this gadget, is interrupting a signal, and clamping it if it goes above a certain set voltage. It has a poteintiometer (sp?) on it, to adjust what voltage it clamps at.

This brings a whole new light to the situation. It could be possible to just unplug the switch (not the safest, but it would work) or ideally place the fuel cut defemcer on the signal wire. However, we need to first find out how the switch works. Does it send a low voltage signal when overboost occurs? or is it a gradual voltage increase like the MAF?

Curious... where in the FSM did you find this tidbit? I'd like to research it in mine too! thanks

#4 All_talk


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Posted 05 April 2004 - 03:28 PM

Both switches are on the front of the passenger side strut post, I did some digging and found the “boost switch” on the wiring diagram but no positive ID on the fuel cut switch yet, but I’ll check wire colors when I get home and maybe I can figure it out.

I downloaded a partial version of the ’89 FSM (PDF file) last week, its in section 2-7, page 80. This section describes the function of all the MPFI components, full of useful bits like… the fuel pressure regulator has a 1:1 ratio set at 36.3psi above manifold pressure and… there is a duty solenoid in the waste gate circuit to compensate for low air pressure at high altitude.


#5 Subarutex


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Posted 05 April 2004 - 03:55 PM

Interesting... I've read most of that too, just missed the part about the fuel cut off switch.

Tonight i'm going to take a look at my vaccum system, i think i have some stuff setup wonky.

#6 WJM



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Posted 10 December 2004 - 10:29 PM

Bump for the new thread about this one.

#7 Skip


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Posted 11 December 2004 - 08:15 AM

Simply put

the FSM is wrong.

I own several
87+ turbo'd cars
and have worked on the entire
boost control system of each.

I have never seen this devise.

It is no doubt left over from the flapper door
(moving air vane) system, which do have this switch.

#8 WJM



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Posted 11 December 2004 - 08:17 AM

I agree.

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