First off, I am lucky enough that my car does not have this problem. This morning however the car wouldn't start and I traced it to a bad ground at the fuel pump connector inside the tank; fixed now. In studying the fuel pump schematic I realized the fuel pump runs at two different speeds.

Low speed is active at idle and under 3K rpms through the fuel pump resistor fed through the main relay. High speed runs through the fuel pump relay which the ecu controls; the fuel pump relay supplies battery voltage to the pump when it is closed. The ecu toggles this relay so the ecu actually controls the fuel pump speeds.

In troubleshooting my problem this morning I realized you can pull the fuel pump relay and the car will run and idle but will not run over about 3k; I suppose it starves from fuel since the pump is running at low speed and not supplying enough volume. With the fuel pump relay pulled the pump can't get 12 volts and switch to high speed; it's forced to run at low speed through the fuel pump resistor. (If you ever have a problem where the car starts, idles, runs but won't go over 3k before popping like all hell, check the fuel pump relay. It could be bad.)

My theory is that the 3k hesitation comes from the pump not getting the line pressure up quick enough and momentarily stalling the engine when the secondaries come online. The improved grounding some use I suppose helps because it could raise the pump voltage slightly; others it does nothing for. To verify my theory someone with a 3k hesitation problem needs to do this below and see what happens:

Put a test light across the coil on the fuel pump relay and mount the bulb where you can see it in the car. Drive the car and see if the hesitation and the lighting of the bulb seem to coincide; actually I guess the bulb would light just a split sec after the hesitation. If they do, pull the fuel pump relay and wire a switch to some spade lugs and push them into the fuel pump wires at the relay socket. This way you can switch the relay (high speed fuel) on manually. You can't just jumper it because then the car stalls at idle; I tried it. With the switch inside the car you can toggle it manually. What you want to do is toggle your switch a couple hundred rpm before you expect the hesitation.

If your hesitation goes away then we are correct; the pump is not switching to high speed quick enough. Therefore the ecu is not engaging the fuel pump relay soon enough and the engine is momentarily starved of fuel since the pump is "lagging" behind the injector demands. This would also explain why going to an aftermarket engine computer solves the stalling problem; it triggers the fuel pump relay sooner.

The problem is that if i am right about all this, I don't know how you could change when the fuel pump relay triggers since it is controlled by the ecu. What I have described could be the cause, but I still don't know how to get around it without being able to somehow change the stock ecu.

If anybody wants to test this out I would be glad to help; I have studied all the schematics and understand the pump's control and wiring. I think this is definately the real cause of the 3k hesitation; not grounding issues.

If somebody already figured this out, then I guess I wasn't in class that day

 

Makes perfect sense to me.

 

If Damon's theory is right, and I'll admit, it sound pretty plausible, then there might be some value in trying different stock ECUs. Basically, if the ECUs are from different cars (and maybe different model years) they might have a different version of firmware on them, which could be just the ticket. Anyone know how to tell which version your ECU is? Also, I think there's some incompatibility issues between the FD model years (like you can use a 93 ECU in any 3rd gen, but 94's and 95's aren't useable in a 93...I dunno).

At any rate, try a different version of ECU and you might find your fix.

 

Great job. I had the 3K hesitation, but I've since added an extra ground, and started my vacuum hose replacement, so I don't know if I still have it or not. Once I get the car back on the road, I'll let you know if I still have it or not, and if so, we can test your theory. Thanks!

 

Good job, Damon, and this is exactly why one of the popular mods for a BPU Supra is switching the fuel pump to full time 12V operation at the relay in the rear quarter panel.

This was discovered for the RX-7 about 3-4 years ago, and we (well, Trev and I) wired our cars for full time 12V operation. You'll "burn up" your pump more quickly (but at 3-4 years expected life, who cares?) but you'll also have all the fuel you need on demand without any hesitation that could risk an engine.

Neither of us had 3k hesitation and I've always believed that the "ground cable fix" was hokey at best. You're probably right about the switching of the fuel pump causing the hiccup, although it was often attributed to the ECU bringing the secondary injectors on-line and fuel rail volume falling momentarily because of it.

If someone bothered to sit down with the schematic for the fuel system, it should not be difficult to figure out how to rig the fuel pump for full time 12V operation without the stalling problem at idle you describe, which we did not encounter. It's been so long that I can't remember what we did, and I don't even have that relay box or the resistor in my car any longer.

 

If that is the problem all you need is a rpm switch to apply 12v at say 2800 just enough to bet the ECU to the punch. Like a msd switch or such.

 

To make the fuel pump run at 12 volts the whole time all you would have to do is unplug the fuel pump resistor and put a jumper across its plug. If anyone else looks at the schematic you'll see why this works.

Guess you could try that too if your car hesitates but like Jimlab said above, the pump life may be shortened.

 

Replacing the chassis ground at the battery may free up just enough juice to fix the problem on some cars. On many others it does not help at all. That's why I come back to the ecu triggering the relay; it synchs with the hesitation.

 

Wow. That is an excellent idea for someone who wants to keep the "stepped" voltage to the fuel pump for longer pump life.

If you disable the ECU's trigger to the fuel pump resistor relay, and instead used an MSD or Accel rpm activated "window switch" (I believe MSD has plug-in modules for various rpms) then you could set it up to trigger full 12V operation at the rpm of your choice, independent of the ECU's operation.

Bravo. I think this is probably one of the most productive threads I've seen in a long, long time.

 

The MSD switch would be very easy to do; just cut the fuel pump relay's coil wires and splice them into the MSD switch (does it trigger with a ground or +12? It would work either way).

First we have to prove this is the problem, if it is the MSD trigger would surely cure it. I can point anybody to the wires you'd need to splice into; very simple.

 

I still think the pump should be at full output before the secondary injectors are brought online anyway.

The MSD rpm switch (PN 8950) provides a ground path, if necessary, or can be used like a traditional relay, passing current through to an output when activated. They have fixed rpm modules (3k, 4k, 5k, etc.), 5-pack module kits (5.1k, 5.3k, 5.5k, 5.7k, 5.9k, etc.), and slightly more expensive "adjustable" rpm modules with 12 distinct rpm settings.





The rpm window switch I mentioned earlier has an upper and lower rpm boundary and is traditionally used for nitrous systems where nitrous activation should be cut out before the car hits redline, so that a missed shift doesn't "zing" the engine...

 

So a question as to the operation of a PFC or other aftermarket computer, do they operate the fuel pump at +12 full time? Or are they just really really good at the switchover? Or I seem to recall that the PFC always had the primaries and secondaries on at the same time? What's that scoop?
~Tom

 

that guy is smart....im gonna print this thread and laminate it

 

Bravo! Even if you're wrong this is still one of the best threads ever. If you're right then you've just fixed the ONE problem that for some was UNFIXABLE (I consider the PFC a workaround). Everyone look! *bump*

 

Great work, DamonB. You're solution seems both logical and probable. I can't wait until someone tries your proposed solution and discovers if it works or not.

Very nice. This IS one of the best threads in quite a while. Archive this sucker.

 

I agree. Great thread. I think that I will have to try switch trick and help prove your idea.

See if you can find another RPM actuated relay that triggers lower than 3K. I'm sure mazda has plenty more crazy methods of control under our hood somewhere.

 

I'm giving this one a go when the body shop returns my car. Thank you.
Cleaver thinking.

 

Damn, what would you people do without me???



Summit Racing - PN SUM-830449

"Our RPM activated switch uses internal DIP switches to select the rpm setting instead of expensive pills or modules which can get lost or vibrate loose. Our switch also has an adjusting **** inside the unit that lets you fine-tune to any rpm from 2,000 to 9,800 rpm. A green LED light lets you know when the desired rpm is reached -- no need to turn on a connected device to see if the switch is working. We also include a red LED power indicator, an internal fuse, and extensive input protection circuitry to keep the switch from blowing up if you connect it to something that draws too much current."

$45.95

 

I sense a nomination for Forum Member of the Year...

 

Wow! Great site! I can tell you this, as a chemical engineer I design plant systems vary similar to how engines run. In the case of fuel delivery it is very similar to bringing in the feedstock to a plant or the main artery...hell I just installed a Coriolis type flow transmitter to characterize flow in the main line feeding to a plant...as consumption increases, a valve modulates open or close depending on demand. Like an ECU we have PLCs that control the functions of the plant that are not dissimilar to the functions of an engine.

It would seem to me that you have a reservoir (gas tank) fed by a pump to an open ended device (injector nozzles). If the process (combustion) is infinitely variable, then the feed process must be equally variable else an upset occurs....in this case "hesitation". In my field, characterization is about rates, temperatures and pressures. If I were to design a similar system I would not have a nearly binary functioning device (2 step motor) integrated with a highly modulated process variable.

Could this existing system be somehow fixed? In an open ended nozzle system pressure is critical to the functionality of the nozzle. I apply pressure transmitter devices (analog signal) to check that the ideal pressure is being maintained and if not an "infinitely" variable pump (pump w/inverter or frequency drive) modulates to maintain the set line pressure. Both the input variable (line pressure in this case) and the out put/response variable (pump rate) is infinitely modulated via analog signals. If the computer cannot maintain the desired process variable it must be physically designed into the piping infrastructure. This can be done by enlarging the feed headers to the nozzles to dampen the variations in pressure/flow. It would be very difficult to have a "smoothed" resultant variable with a somewhat binary device (like a 2 step pump) Can this be done on an FD? Just a thought.

An enlarged header is typical in a system where many nozzles are fed from a main line. This must be approached with caution as consumption is assumed to be equal amongst all of the nozzles. This obviously does not hold true if one nozzles plugs for example.

I think that it is irregular to design the needs of a variable process with a pump device that is not.

Funny thing, I am currently working on a similar problem. We have a main condensate line that feeds to 3 different tanks. A non-inverter driven pump (fixed process variable) feeds 3 independent processes. The condensate is ratioed via modulated control valve to each tank. But each tank has different consumption demands simultaneaously so the valve are always changing slightly. When this occurs, it effects the pressure to the other lines causing those valve to modulate to correct. This creates a ripple effect and disruption to the process and inenvitably the correctness of the ratios to each tank (one of the lines gets starved). In our piping the source header pipe diameter needs to be increased to reduce pressure variations with pressure monitoring device installed. The pump will need to be increased to maintain enough excess pressure to supply all valves when they are fully opened. But this does not correct the a propogated ripple effect when consumption changes in one of the tanks. The only way to defeat this problem is by addressing each line independently. Dedicated pump with dedicated lines to each tank...

man, i've had way too much coffee this morning!

sorry for the ramble

 

Need a write up please with results!

 

I agree with Cover8, I think it IS irregular to design the needs of a variable process with a pump device that is not! That is not the way of Kansei engineering...

 

so you are saying bigger fuel lines will ease the problem or fixed

 

Last year I went through all sorts of things to try and correct what had developed into a major 3K hesitation.

After the standard addition of LOTS of grounds, new fuel filter, cleaning the fuel pump grounds, etc... I did remove the fuel pump resistor and jumped it. Like you did, I went throught the maintenance manual, tested the resistance, it was off a bit, so i just yanked it out of the system. On my car, it did not work - at the time. The hesitation remained.

What did solve it for me was unplugging the PFS PMC. The car was smooth after that. The next day i plugged the PMC back in and the car has been smooth since. NO hesitation anywhere at all.

However, that is not to say the resistor didn't solve the problem. I personally think the PMC learned the hesitation and made it worse. by unplugging it, i cleared the "memory".

I did send my injectors out to RC Engineering, 2 were pretty dirty and one was dripping. I did replace all the O-rings for them as well. I did add, and still have , tons of additional grounds of all gauge sizes. I still have the Fuel Pump Resistor sitting in my tool box and not installed in the car. I did have several emails with dave of KD rotary about the problem at the time, and he also thought the fuel pump resistor idea was a logical one.

so, bottom line, what everyone is saying here just make me that much more sure that the resistor is at a least a significant part of the problem.