Hello.
I have a 97 rx7 RS. It has a catback and is stock with 8,000 miles.
It runs excellent EXCEPT under 1/3 tank of gas.
When WOT in a straight line the car bogs and sputters like fuel starvation but i'm not cornering.

I thought the pump may be weak so i put the tt supra pump from my other FD into the new one and it does the same.

It does this until i fill the tank up over halfway, then it will stop.

Any ideas?
Solutions?

 

Check your afr's if you're wondering if it's running lean. Can't imagine the fuel system is having issues with such low mileage.

Did you do any checks with a fuel pressure gauge?

 

Check the gas tank baffles and be sure the fuel pump is getting enough power. If the fuel pump isn't getting the correct voltage it will kill the pump pretty fast so I'd look into that asap.

 

What ECU are you running?Is it still the stock ECU?

 

I did not check that. I will. AFR I would have to have a wideband for it. I had one lying around but can't find it.

Why would the voltage effect the pump only when the gas tank is getting lower?

I am running the stock (16bit) ECU.
I thought maybe it was hitting fuel cut from too much boost, but that doesn't explain why it only does it with less than 1/3 tank of gas?

 

Don't know I'm just spit balling but maybe the pump is getting hotter or it's harder to gather fuel if it's moving more etc... Again if the pump isn't getting the correct power you will burn it up pretty quick if doing lots of wide open throttle runs as in track use.

I had a car with similar symptoms and it was an electrical issue or the pump wasn't getting enough power (went through 3 pumps before I figured it out).

Good luck

 

i've seen this, its either broken baffles in the gas tank, bad pump/clogged filter(s), or a plugged charcoal canister.

the charcoal canister stores vapors, but it also lets air in, as fuel goes out, and if its plugged it doesn't let air in, the tank will actually draw a vacuum, and the fuel pump wont pump fuel against a vacuum.

 

Fuel pressure gauge will help you with troubleshooting, too.

Checking the filters and baffles is easy and cheap.

 

New symptom: it was just a coincidence that it happened when low on gas. It did it yesterday twice in a row and then no matter how many times thereafter it would not.

It will do this the first two times I WOT it. After that it will not do it again.

I filled the tank in th moning and then WOT it and it did it 2 times with a full tank, and never thereafter.

Sounds like tuning issue and it notices the AF is off and then ECU corrects itself by dumping more fuel. Or perhaps it is not fully up to temp and after two hard pulls it reaches a certain temp. I will drive it on the highway for 20 mins until I'm sure it is at a Normal op temp and then boost it a couple times to see if it is just temp related.

 

What ECU do you have?

I'm thinking you still have some sort of fuel delivery problem

 

Your probably hitting fuel cut.
You should drill out the restriction in the wastegate nipple and add a manual boost controller. (older cars have "pills" built into the hoses, ours have the restriction built into the nipple itself)

Or just drill it out and test if the problem still happens.(which will make it run off wastegate spring pressure, about 7psi) If it runs fine, then fuel cut/overboost is your problem - install manual boost controller or valve, and set it to boost level just under where fuel cut occours

 

O2 sensor issue perhaps. Could be getting bad readings when engine/sensor still cold.

How much boost are you getting? Maybe check map sensor too.

 

Stock 97 16 bit ecu

12.5 lbs - 9 - 12

 

stock ecu doesn't learn, and wont correct itself.

 

That boost pattern remains consistent when the problem is occurring?

 

The reason I was asking what ECU you have (and thankfully its stock) is because you could always use the jumper wire, resistor & LED light trick to see if there are any stored fault codes in the ECU that may lead you down the right path.

If there are fault codes, its easier than stabbing in the dark. Especially since the stock ECU gives out codes easily.

 

Found the issue.
Ecu was cutting fuel after a brief boost spike on primary turbo. It would build boost and shoot to almost 15lbs for less than a second then it would go back down to 12lbs as the RPMs climbed. a good 1.5-2 seconds later the ECU would cut fuel (not a very fast response) so when I saw it cutting fuel it was at a normal boost level.

I put the Stock exhaust and intake back on. It will now only do it if I WOT at a low RPM outside the powerband which gives enough time to build a lot of boost pressure to ~15lbs.

If I am in 3rd gear and floor it from 3000 RPM it will cut fuel between 4000-5000 RPM after overboosting.

If I am rowing throw the gears I can go from a stop to top of 4th at WOT and it will not be able to boost spike.

This means the wastegate cannot bleed off enough boost and needs to be ported, correct?

 

With the stock intake and completely stock exhaust, you should not be seeing 15psi ever on the primary. You're having issues elsewhere. My bone stock rx7 with a bones downpipe has a 12-8-10 boost pattern rock solid. Doesn't matter what RPM I start from.

Matt

 

There are lines going to the actuators on the tubos that have little restriction pills in them.

I'm betting those pills are jacked up or restricting too much and causing the over boost.

I'd replace those hoses with ordinary silicone hose and see if the boost comes down.

If so just buy some new lines from malloy and you should be all set.

 

as above, you can keep your intake and exhaust if you just remove the restriction to the wastegate and add a manual boost controller to set boost at just below where fuel cut happens.... (restriction to the wastegate is either in the form of pills for older models, or built into the nipple on the newer ones)

removing the restriction to the wastegate will result in 7psi boost (wastegate spring pressure), plus any creeping you may have.
adding manual boost controller then allows you to put back "some" of the restriction to the wastegate, allowing you to set it just below where fuel cut happens.

IF you still fuel cut running off the 7psi spring with the boost controller fully open i.e no restriction, then your creeping, and you need to port the wastegate, or add restriction to the intake and/or exhaust.