Hesitation while accelerating
 

Hi guys! I have an 04 MT with 136k on it. When the car starts up, it runs like a dream. I can get about halfway to work and the car pulls hard. Once it is warm (say 7 ish miles down the road) is when it starts acting funny. It will pull hard all the way to about 5500, then from 5500 to 6500 it is really sluggish. Then at 6500 it pulls hard again to redline (though not as hard as it should). I shift and same thing, sluggish until 6500. It started doing this recently and the car gives me 0 trouble with any speeds as long as I'm under 5500. Pulls hard (as hard as they can pull under that RPM lol) and then it's like hitting a wall that the car has to fight though.

I've looked through a bunch of the other threads with similar issues and they don't seem to fit quite right. I have new plugs, my wires and coils are relatively new. The coils looks clean and have no spots on the back. I don't have a cat, my MAF is clean, air filter is clean, etc etc. I was thinking maybe fuel pump or injectors, but why would they work perfectly while the car is warming up and then stop when it's warm? Same thing with the ignition. I thought maybe SSV but again, why would it work perfectly while cold? And the outside temp seems to make no difference. It's been over 100 here and it still runs strong until it's warm. And it's been 70 out and hesitates when warm.

I'm leaning towards pump and injectors, maybe even getting the BHR ignition, but I don't want to drop $1000 in parts and have it be something crazy. My current plan is to get my injectors and fuel rails cleaned, while the engine is open I'll clean and inspect the SSV and other goodies. If that doesn't solve it, fuel pump and lastly, ignition system.

Any ideas, do's or don'ts before I start working on it? Maybe point me in a direction to check first? Thanks a ton guys. :)

Glad to know it's not something obvious since I haven't been slammed yet. haha I still haven't been able to pinpoint, but I'm planning on starting with my fuel, since it's all still factory at 130k miles on it. If anyone has advice, please don't hesitate to let me know. :)

It is possible that your fuel pump starts to give out after it warms up. That has happened in a previous car I have owned. You would have to check your fuel pressure to know for sure. Do you ever track your car? You could always throw in a Walbro pump for $100 bucks.

+1 on fuel pump and filter...

I could see that. The pump losing pressure when it gets hot. That would explain why it starts to accel again at 6500, when the secondary injectors kick in. More fuel but not enough pressure to run at 100%. I'll look into it. And I used to autocross one or two times a month, but haven't for a few months. Hard to do in the heat. haha

I need to check my AFRs too. Gotta drag my AP from the closet of doom and see what it's doing.

fuel pump...i replaced mine with walbro255

You can also pick up another OEM pump without the assembly on ebay for $50 if you don't want to mod the fuel assembly, although it isn't hard to do.

When the engine hesitates under acceleration, does it cut out abruptly before it kicks back in, or is it just mildly hesitant? My 04 GT has had sudden power loss at ~5500 then resumes to redline, after warming up for a bit as well.

It doesn't cut, it just hesitates hard. It accelerates fine and when it hits 5500 it just slows waaay down and then at 6500 goes back to accelerating like normal to redline, I shift and the process starts again. Though from 6500 to redline isn't as fast as it normally is. When cold, the accel is much faster all the way through the RPM band. When the car's warm, the low end is pretty similar and the 6500+ is a bit sluggish, though not nearly as sluggish as 5500-6500. It's like the needle is moving quickly and then slowly to a crawl.

I read a bit earlier, to mod the assembly I just need to take a soldering iron and melt the blow off cap to the assembly, right? So that the increased pressure doesn't shoot it off?

Theoretically I should be able to get the pump next weekend and I'll have an update shortly after that. I'm trying to get the time to check my AFRs so I can post them but my school and work schedule are right on top of each other right now. Hopefully this weekend!

Yeah, at least that is the way I did it. I'm sure there are other creative ways to melt the top. To me the hard part was getting the fuel assembly ring off without breaking it and disassembling the fuel assembly. Just take your time, the plastic is old and has been sitting in gasoline and it doesn't take to much to break it.

Thanks for the tips. I ordered my pump last night. Should be able to pop it in next weekend and we'll see if that solves it!

Do you know when is the last time your replaced your coils and plugs?

Ok, replaced the pump. Starts much smoother, idles really nice, low end has a big bump in power, top end too. Got down the road, hit 100 no problems and I was loving life. Got a few miles down the road, turned around and did it again, same damn hesitation. I don't know what to do.

My plugs were done maybe 8k ago, coils probably closer to 20 or 30, but they all look really good. I don't know what to do.

My next guess is to have my injectors serviced and clean my UIM and valves while I have everything apart. My last resort is the BHR ignition system. If it's none of the above, I'm stumped. lol But cashflow isn't that great so it's baby steps for now.

Ok, after driving it a bit yesterday, I realized that there was something definitely wrong. The top fuel line is leaking like crazy. There doesn't appear to be any damage to it and I have the little green basket thing lined up in it, I can't figure out what is wrong. Thanks for any help guys. I've looked at several pics and everything looks right in mine.

After digging through the threads, I think I'm going to do what a few other people have tried. Cutting off the quick connecter and clamping the hose directly to the nipple using o clamps. Any thoughts?

Ok so I cut off the end, replaced it with a hose that fit over the barb on the fuel pump assembly. Used o clamps before and after the barb and on the other end. Worked like a charm. No more leak, consistent pressure, no issues. However, the initial problem still stands. I would like to attribute my valves, but it doesn't make sense that it runs perfectly until it gets up to operating temp. But most signs point to the valves, I just worry that it's over my head. :/

SSV is pretty easy to clean and get to. you just need a uniswivel to get to the back bolts on the upper intake manifold to remove it.

That's reassuring. :) I'm going to have a mechanic friend of mine remove my injectors for cleaning. Doesn't the process for getting to and removing the injectors open up the path to clean the intake valves? I thought I read that somewhere and if it's true, that will make life sooo much easier.

hey ! sorry for bumping such an old thread. but recently i have the same problem as well.. At about 5000 rpm to 5500 rpm when i accelerate there is no problems below 5000 rpm , but as soon as i hit 5000 rpm the car hesitates to move forward and there is a pullback feeling. After exceeding 5500 rpm the car starts accelerating well again all the way to red line.. this is most apparent at 1st and 2nd gear. Sometimes when im accelerating and holding the clutch very slightly the hesitation at 2nd gear doesnt seem to be apparent. But in 1st gear it always hesitates and makes it very hard for me to accelerate. My Rx8 is a 2003 manufactured JDM model hi-power version. Any ideas on what is the problem? thanks!

just had this problem with our race car and i have found our fuel rail regulator has failed had 4.5k -6k hesitation over that was good power.we are using the walbro pump and after 2 races of 17 hours the regulator has failed on the secondary rail

Was the issue resolved? I'm having this problem too

was this problem intermittent? sometimes i get it between 2-3k, sometimes from 5-6k, and not consistently...