DamonB

At this point it's great we all agree, but until somebody can prove my theory right it's all just guessing.

I am sure somebody is working on it already; I know it drives a bunch of you crazy.

poss

iTrader: (5)

during the 3k hesitation, does fuel pressure drop from the pump? i don't have a guage, so i have no way of knowing.

i hope this is another "fix" that i can try!

btw, is this resistor located on or near the pump?

silver93

no , the resistor is in the engine compartment.
it is about 2 inches x 2 inches, aluminum, sits toward the bottom of the compartment, near the driver's side shock.

DamonB

Yeah, like silver93 said. If you have cruise control you'll have to move the control mechanism; the fuel pump resistor is under it.

cover8

Certainly, all theories need to prove true...a good example is the use of barometric tanks in multi-stage evaporation. The barometric tanks acts as a collection point to absorb "burps"or upsets in the vacuum system. This way the vacuum in the evaporators remains equistatic (steady-state). Upsets in the vacuum would create upsets in the evaporation process.

This would apply to the "hesitation" theory. The fuel consumption demand over the operating range may be characterized linearly, quadratically or some nth order polynomial but what is important is the supply rate be similar in order as the consumption rate else an upset will occur. The physical process may design in a dampener to absorb this upset i.e. large supply header. I do not think this exists on an FD.

I do not understand engine ecu's very well. Fuel management seems pretty straight forward. If the fuel supply cannot change with demand there will be a disruption. In the plants we have trends to monitor the process variables. It would be considerably helpful to transpose the consumption rate line with the pump output (current, rpm or flowrate) line and the theory would prove by inspection.

neo_omega

someone should put this in Archive section.

j9fd3s

iTrader: (3)

we actually tested this in my car and found 2 things
1) it switches over at 1psi of boost, IT IS NOT TIED TO RPM. we got it to switch over at 2k rpm and up to like 5k
2) it stalls when you take the relay out because it sets a code with a limp home mode, it is of course one of the codes that doesn't turn the light on....

mike

poss

iTrader: (5)

so in order to bypass it, i just need to short the plug connecting to the resistor?

Mr rx-7 tt

iTrader: (5)

The Supra TT has a similar problem and here is an article one of the supra guys posted to help me out:

The stock fuel pump on the twin turbo Supra has two modes of operation. There is a high speed mode for high engine demand, and a low speed mode for cruising. The fuel pump ECU(seperate from the main ECU) receives information from the main ECU which determines which mode the fuel pump should be operating in. Various sensors come into play to determine when high and low speed operation should be used.

Under idle conditions and cruising, the fuel pump ECU sends a reduced voltage output to the fuel pump(9 volts), and the fuel pump operates in "low speed". When engine sensors determine a high engine load, the fuel pump ECU will send a full 12 volt signal to the fuel pump, kicking it into "high speed" so that it will supply more fuel.

The Problem:

When modifications are made to the car to increase boost pressure and engine breathing ability, some owners have experienced detonation around 4000 rpm, as the 2nd turbo builds boost and comes on line. Usually, this detonation disappears above 5000 rpm. The problem is, the fuel pump is still in "low speed" mode around 4000 rpm's, but the performance modifications have increased fuel demand to the point of "outrunning" the low speed operation. By 5000 rpm, the fuel pump ECU has caught up and switches to high speed operation and all detonation disappears.

The Solution:

There is an easy way around this. You can wire a 12 volt signal directly to the fuel pump, effectively bypassing the fuel pump ECU so that the fuel pump is always in high speed operation. The TT Supra is one of only a few cars that has this dual mode of fuel pump operation, whereas most cars always have 12 volts to the fuel pump. This mod is completely safe. You are basically just turning your complicated fuel system into a basic, Camry style fuel system. You may shorten the life of your fuel pump from say 20 years down to 18 years, but nothing to worry about in the short term. I've personally had mine hooked up this way for over 2 years now without any troubles.

The fuel pump ECU is located in the trunk area(Fig 1). Pop your hatch, and look for the trunk courtesy light on the driver side plastic paneling by the rear shock tower. The fuel pump ECU is bolted to the body, under this panel. To access, pull back the carpet. You will expose a black piece of styrofoam on the left side of the spare tire. Remove the spare and pull out this black styrofoam, it is only velcroed in place. Once the styrofoam is out, you can reach up under the plastic and feel around for the ECU. It is about 4" x 6" x 4". There will be an electrical plug on the side of the ECU closest to the tail light. Unplug this plug. There will be enough slack in the wires to pull the plug below the plastic paneling for easy viewing. There should be a blue wire with an orange stripe. Note that for 1997 this wire is solid blue, on 1998 this wire is black with a red stripe. - verify which wire applies to you before you proceed. This is the 12 volt power supply for the fuel pump ECU. It is only 12 volts when the ignition is in the "on" position and has a fuse in the closer to the battery. This will be the new supply for the fuel pump. There should be a blue wire with a red stripe. This is the output signal from the fuel pump ECU to the fuel pump. Cut the blue/orange wire and the blue/red wire and connect these two together. You now have 12 volts wired directly to the fuel pump. use electrical tape on the two wire stubs still connected to the fuel pump ECU so they can't short out on body ground. Make sure you use good quality electrical connectors so the fuel pump doesn't see an intermittent signal or short out on body ground. Plug the connector back into the Fuel Pump ECU.

jimlab

Great, now everyone knows how to perform the 12V fuel pump mod on a Supra. That really helps tremendously with resolving the 3k hesitation of the RX-7. Thanks for posting that valuable information...

BTW, I mentioned the 12V mod for the Supra in the first sentence of my first post on the first page... welcome to yesterday.

Mr rx-7 tt

iTrader: (5)

Hey Mental Midget, The point is the supra also has a 2 speed set up and they hot wire the pump. Why don't you go post and ask everybody your age or what color underwear you wear or some other dumb post....

Mr rx-7 tt

iTrader: (5)

I guess it must be relevent then? Why is it relevant if you post it but if someone else posts it it isn't..? Thanks for showing us you are an idiot...

jimlab

Flattery will get you nowhere.

While it is true that the Supra also has a 2-speed fuel pump control system, the details of the modification of that system are relevant only to the Supra, and have no bearing whatsoever on the components, component location, or operation of the RX-7's 2-speed fuel pump control system.

I would have assumed that someone as highly intelligent as yourself could see that posting the full instructions for modifying the Supra system in a thread concerning resolving the 3k hesitation issue of the RX-7 might be considered by some to be, shall we say, a complete waste of time?

Of course you're no stranger to posting information of dubious value, from what I've seen in the past. Keep up the good work.

Brian P

.

Would a 'surge' tank work to keep extra fuel on hand? Like the ones for oil, when the preasure drops it compensates?

Quote from www.summracing.com

The Moroso Oil Accumulator just might save your engine from damage by giving you 1 1/2 extra quarts of oil in reserve. Plumbed into the pressurized side of your oiling system, the Accumulator uses compressed air to force its oil reserve into your engine when the oil pressure drops below normal. When the pressure gets back to normal, that reserve oil is forced back into the Accumulator, ready for the next emergency. It can help build horsepower by allowing you to use less oil in the crankcase, reducing crank windage without the danger of engine damage due to lack of oil. You can even use the Accumulator as a manually operated pre-lubing device during startup. 13 in. long x 3 in. in diameter. Includes rubber-lined mounting brackets.

rotary-tt

I hate to burst everyone's bubble, but this info has been out for quite a while. Take a look at Steve Cirian's site regarding the 3k hesitation http://www.scuderiaciriani.com/rx7/3k_hesitation.html PS - there is a lot of good info on this site: http://www.scuderiaciriani.com/rx7/

I don't think anyone has a positive answer to this problem. Lot's of conjecture but no fix. I've never heard of anyone fixing it for sure unless they got a PFC. I've heard that fixing the fuel injection ground under the intake may be a fix. I've fixed all other grounds with no luck...

spooledUP7

Other than the ugliness that occurred a few posts ago, this thread still rules.

I have not tried to perform the suggested test, but I will do some investigation on the two-step pump soon. I had some hardcore hesitation yesterday at light throttle which reminded me to come visit this thread again.

Cover8 - Dude, I had no idea you had it in you. Needless to say, you lost me at hello. Just kidding, great writing?my brain hurts, but great information anyway. Oh yeah, thanks for the door handles, they look bitchen.

93FD3S

So has anyone tried this mod yet? (bypassing the fuel pump resister and running the pump full time high speed) Please post results...Thanks..Tony

rx7will

iTrader: (6)

The rx7 TII have the same type of dual voltage fuel pump, its suppose to save gas and reduce noise. What i did was pretty simple I took the power wire to the fuel pump and i cut it i used the end from the ecu to triger a good quality relay and i ran a wire from the battery to power the relay, and i power the fuel pump from the relay. Basically i'm using a relay to feed direct battery voltage to the fuel pump. The reduce voltage is enough to switch the relay on. Its very simple to verify at idle because when the fuel pump is running it will be louder than before. The only thing that i noticed with my tii was that after doing this direct voltage fuel pump and running a walbro 255 fuel pump and the stock fuelpressre regulagtor, the car would flood when starting because during cranking i would see 60 psi of fuel pressure. But after putting in an aftermarket adjustable regulator it was all good. I guess the stock regulator wass too small to bypass amount of volume from the direct powered walbro pump. I havent done this to my fd yet.

93FD3S

Anyone else do this mod to there FD?

bureau_c

There's something about this that doesn't make sense to me, but maybe my car is a bit different (although not really from what I have read from others). I get the 3K rpm hesitation only when the engine is cold. Once the car warms up, or I jam the gas and blast through the hesitation once or twice, it is gone. There is a much smaller, barely noticeable hesitation under light throttle still, but nothing like the cold engine hesitation, and nothing at all under WOT. If this theory is correct, why would I not get the major hesitation all the time? Are you guys also only seeing it prior to being warmed up, or do you see it all the time?

jds

Carbomer90

I too only have the hesitation when cold after warm up it is completely gone.

radkins

Me too, but the engine is using alot more gas when cold. It almost seems like it is getting confused and switching back and forth rapidly until it sees a definite reading either over or under the preset rpm.

BATMAN

hmmm the last two posts sound like the butterfly flaps in the throttle body.. i maybe wrong about that.. but isnt that the purpose of the flaps.. ?

bureau_c

Not sure why trying to use more gas would make the problem more obvious. It almost seems like the opposite would be true, at least if the ECU puts the fuel pump in 12V high-flow mode when cold (which I have no idea about...anyone?)

jds

KoonAss

this problem is the opposite. there is a constant and sometimes severe hesitation or bogging down until the turbos spool up(which doesnt seem to happen sometimes). I was told that my "speed switch" needed to be replaced since the speedometer stopped working last week. It's an A/T and the tranny seems to shift too early keeping the rpms low. help!