3800 rpm wall/ hesitation

[Apex944]

So, I have been trying to hunt down the reason for why I get an early fuel cut between 3500-4000 rpms. If I go just partial throttle, not even 1/4 throttle, it'll continue on up to 4k rpms and on. But if I give it 1/4 or more throttle it hits fuel cut around 3500-4000 rpms and falls on its face until I let off the gas.


It idle's perfect, rev's perfect sitting still all the way to redline, drives perfect under 3500 rpms.

I have a few idea's and have been doing tests with my DMM and other things but I just can not figure it out.

I figured it was the MAP sensor so I tested it.
The Black/White wire reads 12.2 volts
Brown/Red wire 2v at 3.9 in Hg
Brown/White wire 4.99v
Black wire is ground

I think I may have been doing the test incorrectly but it says to check pin D on the connecter with KOEO and the 3.9 inHg on the sensor.

I even hooked up an RB FCD to my map sensor just in case it was faulty, that way it would be bypassed. But it still has the same issue.


My TPS seems to be good as well. I hooked it up to my O-scope and it doesnt have any drop offs or spikes anywhere and I set it to 1k ohms at rest and it hits 4.4k ohms max/WOT.

My fuel injectors were just cleaned.
My fuel pump, fuel sock, fuel filter are all brand new.
New spark plugs, wires, coils are all good.
Timing is close but I have the wrong pulley so I cant set timing just yet.
Grounded the MAP sensor ground and pin 2C on the ECU which is the sensor ground.

I gutted the main cat converter and checked out the resistor pack.
The car does run better with the gutted cat, and the resistor pack is good but the 3800 rpm wall is still there.


My AFM had 2.5 k ohms of resistance on one of the tests for the pins. Should have only been 200-600. Pins E2 and Vs.
I put another AFM on it, the correct one I need of course, and made sure it was to specs.

Still has the hesitation.

I pulled the vacuum to the map sensor and capped it off to simulate a load on the engine and to see if the 3800 rpm problem is secondary injector related, and of course it is.

At exactly 3800 rpms sitting in the driveway the car bounces off of it like a rev limiter and it leans out everytime it hits, although the misfire could be causing the lean condition.

I checked for voltage coming out of the resistor pack- a little over 12v

So I removed the UIM and checked out the wire colors to the injectors.
So far so good, everything is plugged in correctly according to the haynes wiring diagram. The FSM doesnt specify the colors all that well....

I tested for voltage, yep they all have a little over 12v.

Tested for continuity from the injector plug to the computer plug, yep they all have continuity and are the correct wires for the correct pins and only have .3 ohms of resistance on them.

I re-checked the resistance of the injectors and they are all the correct low impedence injectors.

I hooked up an LED to 12v and grounded it to one of the secondary ground wires off of the ECU. It lit up just like it should when it hits 3800 rpms and it bounces off of it until I let go of the gas.
Which means the computer is telling them to fire so they should be working.

I put a needle into each ground wire off of the ECU for the injectors and grounded it out intermittently with the ignition on and I heard clicking come from each injector which means theyre working fine for the most part.

Anybody have any idea's?
I havent gotten any replies to my thread from "the other forum" for a few days now....

I was thinking possibly too much fuel when the secondaries kick on due to my walbro 255 pump?

Thanks for any help

[Phoenix7]

damn, good job on the work so far. you say you get 4.4K ohms at WOT but the FSM says it should be at 5K ohms +/-1K ohms but that may still be causing the stumble. This is assuming you have an S4 since you didn't mention narrow or full range (If not then you're off from the 6Kohms specs).

Have you double checked to make sure the fuel pump is getting full voltage?

IS your timing off? You say you have the wrong pulley, does this mean the marks are off? Have you set the motor to TDC and remarked the pulley?

Your secondary injectors are electrically within specs, is it possible that they're clogged though? It's a long shot but who knows.




It IS possible that you're just flooding the car when the secondaries come on. It only happens when idleing, right?

Good luck man. I'll think about it and see if I have more questions or ideas.

[Apex944]

Originally Posted by phoenix7

damn, good job on the work so far. you say you get 4.4K ohms at WOT but the FSM says it should be at 5K ohms +/-1K ohms but that may still be causing the stumble. This is assuming you have an S4 since you didn't mention narrow or full range (If not then you're off from the 6Kohms specs).

Have you double checked to make sure the fuel pump is getting full voltage?

IS your timing off? You say you have the wrong pulley, does this mean the marks are off? Have you set the motor to TDC and remarked the pulley?

Your secondary injectors are electrically within specs, is it possible that they're clogged though? It's a long shot but who knows.

It IS possible that you're just flooding the car when the secondaries come on.

Good luck man. I'll think about it and see if I have more questions or ideas.

Yeah, I forgot to mention it is an S4 T2 so it has the 1 " on/off " TPS sensor.

I figured the TPS is fine due to some information I found while searching for this problem. I've seen other members who have only 4k ohms for WOT and others around 4.5 ohms WOT on their S4 TPS sensors and dont have any issues. I have tried a few TPS sensors and nothing has changed.

Im going to double check my fuel pump voltage after I repair my intake manifold leak tonight. Just hook my DMM up to it and watch it as I drive around and see if it drops off any where.... I know the DMM isnt that great at response but it's the best I have at the moment and should be good enough for fuel pump voltage watching.

As far as timing goes.... I am 99% sure I have the wrong pulley on my car.
For starters when I line the CAS up, yellow timing mark, etc.. for setting up the CAS and drop it in, the car will not start at all. I also tried setting it again last night and my LED setup I made for watching my secondaries flashes as it tries to start so it must be off....
I have spent 3 hours trying to set my CAS with no luck, so the pulley is the only reason it is doing what it's doing.
I then just went 1 tooth at a time until it ran decent enough to drive.
Then at rotorfest a few weeks ago Icemark told me my CAS was 180 out so I flipped it and it ran even better and it was retarded like it should be again.

I know im close to the correct timing, but might still be off.

I keep forgetting to try to set it to TDC and remark the pulley, i'll do that tonight as well.


Im waiting on a pulley to get here that another member is supposed to be shipping me but so far it doesnt look like its been shipped out yet...

Anybody got one laying around that I could buy for like $20 or so shipped?


Thanks for the help

[vex]

Originally Posted by Apex944

Yeah, I forgot to mention it is an S4 T2 so it has the 1 " on/off " TPS sensor.

I figured the TPS is fine due to some information I found while searching for this problem. I've seen other members who have only 4k ohms for WOT and others around 4.5 ohms WOT on their S4 TPS sensors and dont have any issues. I have tried a few TPS sensors and nothing has changed.

Im going to double check my fuel pump voltage after I repair my intake manifold leak tonight. Just hook my DMM up to it and watch it as I drive around and see if it drops off any where.... I know the DMM isnt that great at response but it's the best I have at the moment and should be good enough for fuel pump voltage watching.

As far as timing goes.... I am 99% sure I have the wrong pulley on my car.
For starters when I line the CAS up, yellow timing mark, etc.. for setting up the CAS and drop it in, the car will not start at all. I also tried setting it again last night and my LED setup I made for watching my secondaries flashes as it tries to start so it must be off....
I have spent 3 hours trying to set my CAS with no luck, so the pulley is the only reason it is doing what it's doing.
I then just went 1 tooth at a time until it ran decent enough to drive.
Then at rotorfest a few weeks ago Icemark told me my CAS was 180 out so I flipped it and it ran even better and it was retarded like it should be again.

I know im close to the correct timing, but might still be off.

I keep forgetting to try to set it to TDC and remark the pulley, i'll do that tonight as well.


Im waiting on a pulley to get here that another member is supposed to be shipping me but so far it doesnt look like its been shipped out yet...

Anybody got one laying around that I could buy for like $20 or so shipped?


Thanks for the help

Have you tested your secondaries per FSM procedure (mechanics wire?) I'm thinking since it's a problem with the secondaries you may actually be able to physically see the secondaries firing. If you're getting fuel out of the injectors then the problem is in the chamber and it's not atomizing correctly.

So far from what you've told us it seems like you've tested a majority, if not all, the sensors that would have a bearing on this problem. I would look back at the secondary injectors and do the FSM procedure.

[My5ABaby]

My guess is the ground wire on the boost/pressure sensor isn't grounding right. That's the cause on the 86's which was supposedly fixed in 87+. However, they fixed it by adding another ground. If that ground has gone bad after (in your case) 20 years, that could cause a stumble.

I had the stumble from ~3600rpm to redline and regrounding fixed it.

[TehMonkay]

Switch out the injectors.

[WE3RX7]

How worked over is this car? Motor swap, etc done by previous owner when you picked it up?

The standard "stumble" that many FC owners had at 3800rpm was the bad ground as was mentioned above - you're describing something different.

When you hit that "wall" does the car just simply bog down or shut off?

What color are the secondary injector tops? Are you sure they are S4 T2 injectors as appropriate for the car? Does the check engine light come on at all during any of the symptoms?

I'm just trying to weed out a few things in my mind - its just all to common (especially these days w/ all the "drifter" scenes running around) to pick up a frankenstein car that somebody butchered and now you - the good owner, is stuck with figuring out what they did!

[Apex944]

Originally Posted by WE3RX7

How worked over is this car? Motor swap, etc done by previous owner when you picked it up?

The standard "stumble" that many FC owners had at 3800rpm was the bad ground as was mentioned above - you're describing something different.

When you hit that "wall" does the car just simply bog down or shut off?

What color are the secondary injector tops? Are you sure they are S4 T2 injectors as appropriate for the car? Does the check engine light come on at all during any of the symptoms?

I'm just trying to weed out a few things in my mind - its just all to common (especially these days w/ all the "drifter" scenes running around) to pick up a frankenstein car that somebody butchered and now you - the good owner, is stuck with figuring out what they did!

The car was pretty messed up when I bought it a little over a year ago.
I myself went through the entire engine wire harness and re-wrapped and repaired it all to a useable condition. I also rebuilt/modified/upgraded and installed a 13BT Series 4 engine into the car. Was well worth the $900 I paid for it though haha. Spare vert rims, parts, most of the engine parts were re-usable, drivetrain is all good, suspension is good, only a few dings/dents here and there etc...

When I hit the wall it backfires/misses/shakes like it's running out of fuel.

It has the correct injectors but does not have a check engine light because it's an '88 S4 which did not come with a check engine light.


I found the problem though

Me and another member who lives near by came up with the idea of grounding out the secondaries via the wire harness off of the ECU while the car was running and to feel/hear/see the a/f ratio of the car.

So car running, holding it at 1500 rpms, tap the front secondary ground wire on/off of a good ground. Yep, it bogs down and goes rich. Do the rear secondary...nothing happens....try again....nothing happens....clogged injector in the rear secondary.

Going to go have a nice talk with the guy who supposedly cleaned them for me....heck, I dont really feel like trying to have him do them for a 3rd! time so I might send them out to witchhunter.... Long story short when he "cleaned" them the last time he said his machine could not fully flow them but they were "good"... WTF he was on I have no idea because if his bench cant flow 550cc's then he shouldnt even be doing injectors.

But yeah, thanks for the help everyone.

[Phoenix7]

at least you found the problem.

[Boostmaniac]

Sounds like it is time for some 720's and some Rtek

[Apex944]

Originally Posted by Boostmaniac

Sounds like it is time for some 720's and some Rtek

I wish I had the money for that....
Eventually I'll go to an Rtek 1.5 and then on to the 1.7 but for now I have to use the stock fuel system and stay under 8 lbs of boost. Also have to get it to pass smog next week too haha.

[vex]

Originally Posted by Apex944

I wish I had the money for that....
Eventually I'll go to an Rtek 1.5 and then on to the 1.7 but for now I have to use the stock fuel system and stay under 8 lbs of boost. Also have to get it to pass smog next week too haha.

Or just save your money and pick up an Rtek 2... I'm pretty sure each one of those Rteks cost 100 bucks (maybe 50, not sure now). So you'd be half way there. Spend 400 instead of 600 ya dig?

[Boostmaniac]

2.1 all day.

Rumor has it that they WILL be working on a MAF delete and a 3 bar MAP sensor update....

:::laughs maniacally:::

[Apex944]

Well, im just looking for a little adjustability which is why I want the 1.5 and then the 1.7 . It only costs $10 or so to upgrade from 1.5 to 1.7 . That'll let me have some fun for a while and keep things simple.

The 2.1 is cool, but if im going to spend close to $600 on something that is somewhat adjustable I might as well go to the next level and get the Apex'i Power FC. I know a few people who are running the Power FC and they love it.
Plus the Power FC uses a 3 BAR Map sensor in place of the AFM which is a big plus for me. Barn door AFM's are garbage.
The 2.1 Rtek still uses the AFM for an airflow input, which I want to get rid of ASAP.

[vex]

Originally Posted by Apex944

Well, im just looking for a little adjustability which is why I want the 1.5 and then the 1.7 . It only costs $10 or so to upgrade from 1.5 to 1.7 . That'll let me have some fun for a while and keep things simple.

The 2.1 is cool, but if im going to spend close to $600 on something that is somewhat adjustable I might as well go to the next level and get the Apex'i Power FC. I know a few people who are running the Power FC and they love it.
Plus the Power FC uses a 3 BAR Map sensor in place of the AFM which is a big plus for me. Barn door AFM's are garbage.
The 2.1 Rtek still uses the AFM for an airflow input, which I want to get rid of ASAP.

it was 400 last I checked.

As for the delete it should have been here ages ago....