I've got a big problem. My RX7 (a S4 TII) is hesitating at exactly 5000rpm. I had hesitations at 3800rpm, too but I regrounded the whole ECU and they went away. Only the one at 5k still is there. Because of that I changed the plugs and plug wires but nothing helped. I also checked the tps and set it up correctly. Still no change.

One of the old trailing plugs looked a bit weird. While the one of the rear rotor was totally black, the one of the front rotor nearly looked like a new one. I could see the metal of the plug nearly everywhere. Only a really small piece of the plug head was black. The leading plugs both looked exactly the same. Both were brown/grey and looked like they did a good job.

Could my trailing coil cause a hesitation at exactly 5k rpm? It only happens then and when I rev it higher it's like nothing ever was wrong. I nearly tried everything but changing the coils because they are damn expensive in Germany even when used. I would do that, too but first I want to know if I'm on the right trace.

By the way the car is totally stock and the engine is good so far. Compression at all chambers, not burning coolant and so on.

If I don't find the reason for that hesitation soon I'm going to freak out. I got that car 5 months ago and spent every free minute in repairing it and it's still not running like it should.

 

hesistations are almost always fuel related.

if the fuel filter hasn't been changed, change it. if it's still there check the voltage to the pump itself. if those 2 things are ok then have the injectors sent out to be checked and cleaned.

 

The fuel filter was the first thing I changed. I changed the pump, too. Now a Bosch 044 pump is working inside the tank. It's still strange that the hesitation only appears at exactly 5000rpm and nowhere else. I could change the pump back to the old one just to be sure it isn't the pump.

 

What other modifications have you made to use the Bosch 044? Is everything else stock??

 

the 044 is likely going to push too much fuel through the injectors causing a rich condition. without a wideband you can only guess by throwing upgrade parts at the car.

 

Yep everything else is stock. I just put the Bosch pump in because I have some connections to Bosch and got one almost for free. The pump the FC had when i bought it wasn't original, too. It was a BMW E46 M3 pump.

BUUUUUT the FC is equipped with a mechanical fuel pressure regulator. Schouldn't that thing be able to compensate the bigger pump?

 

you would think so, but once you start pushing enough volume the short answer is: no, high output pumps will overdrive the stock FPR and pressures will start to creep upward.

if you're curious put a pressure gauge on the line from the filter to the engine. should be no more than 32psi idling and 40psi with the vacuum line to the FPR disconnected.

it sounds like your pump is masking other fuel system issues. perhaps the injectors are in fact dirty, or you have a vacuum leak, or the airflow meter is on packing up to go on vacation.

 

Hmm ok so what would you suggest? Get the original pump or get a pressure regulator? By the way I had many vacuum leaks and even clogged injectors but the injectors are cleaned and the vacuum-stuff is new.

 

better to try and find a pressure gauge to put on it versus swapping parts constantly. rule out one thing after another.

 

How much pressure should i get behind the filter?

 

listed above.

 

Oh sorry.

 

I tested the pressure with a gauge today and found out that the pressure is 44psi when idling. Much too high. I got an adjustable pressure regulator today. Only problem now: How do I disable the original regulator? I would need a fuel hose connector instead of the regulator but where could i get such a thing? I guess the diameter of the thread would have to be pretty big.

 

you can tap and thread the rail for an adapter fitting to either a barb or an AN adapter to run the line to the regulator.

 

Before changing or adding on another FPR you might want to check to see if the vacuum hose to the FPR has vacuum on it with the car idling as it should. If it doesn't then the fuel pressure increases by about 10 psi.

 

I changed back to the stock fuel pump today to look if the hesitation is caused by the higher pressure. It wasn't.

The hesitation is still there. Nothing changed. I also set up the TPS again. It was at 700 Ohm when idling. Set it to exactly 1kOhm. After that I tried to measure the resistance at full throttle but i wasn't able to do that. When i opened the throttle completely the resistance wasn't maesurable anymore because it got too high. Might be a broken tps. I'm going to repeat that with another meter tomorrow just in case mine could be damaged.

Might that cause hesitations, too?

 

check ignition timing as well. is there any abnormal noise associated with the hesitation like a popcorn sound? 5k is generally the range where the engine makes peak torque and everything has to be ideal.

 

Did you check the fuel pressure w/the stock pump? If you didn't measure the pressure and assumed the pressure would be lower, well that is why it was suggested to check to see if the FPR solenoid was working properly as it either provides vacuum to the FPR or it doesn't, and in the event that it does not, well the net result is a higher fuel pressure by an additional 10 psi.

 

I measured the pressure. It's lower again.

No. There is no popcorn sound. It's just like something is completely interrupting the ignition. Something like a fuel cut. At least it feels like one.

 

what's your car boosting to and does it have anything like a fuel cut defenser?

 

It's a stock S4 RX7. The pressure get's near the end of the scale of the stock gauge but never higher.
That hesitation never shows up in the first gear by the way but the higher the gear the worse it gets. I guess it is connected with the load. The more the engine has to work, the heavier the hesitation gets.

 

and more load means higher boost levels.

what other modifications do you have to the car?

by hesitation i figured a slight dip in power, fuel cut is more like hitting a brick wall. is that what it does temporarily?

 

By "stock" i meant no modifications at all. The only modification was the new fuel pump which i changed back to a stock one today.

Hmm... hard to explain. In second gear it's just a slight dip in power, in 5th gear it's like a fuel cut but i still can rev the engine higher after that happened once.

 

don't really know then.

try checking for codes.

 

Did that. No codes at all. Could a missing spark from a trailing plug cause that? Some days ago i changed the plugs and one of the old trailing plugs looked like it wasn't used as much as the other one. It really looked different. One was completely black and on top of the other one you even could see the bare metal.