theory

iTrader: (5)

Background info:

87 S4 NA
Just finished S5 intake swap
Idles ~800rpm smoothly, drives smoothly until 4k, then it's like a rev limiter turns on

I searched and here found some stuff to test, here are my results:

Resistors in resistor pack are all ~6.5 ohms
Primaries and secondaries were all within spec for resistance as well
I've re-grounded the grounds at the ECU, re-grounded the grounds in the engine bay (including the ecu ground on the UIM)
Boost sensor vac line has the orifice
I've removed and plugged the boost vac line and used an LED in the injector clips and when I rev to 3800 it turns on solid and then pulses

My only thought is a clogged injector. I applied 12v to each injector, they both "click", one seems more prominent then the other though, it's actually hard to tell if the one is clicking or if it's the "crack" of the electricity.

During the swap, I was installing fresh O-Rings/grommets on the injectors when one of the pintle caps broke. I had 2 injectors lying around in a spare intake manifold of unknown condition. It was within spec for resistance so I installed it in the secondary rail for easy removal if it was bad.

Before I replace the secondaries, is there any other possible explanation? I'm just trying to cover all my bases as I can't afford anymore downtime.

KompressorLOgic

iTrader: (6)

id belive it is a clogged injector as you say, if any of them is in an unknown condition, id at least get thje flow tested or swap to a know good one.

HAILERS

If your real easy on the pedal, will the car go over 4 grand? Like up to 5-6-7000?

Pressure sensor actually getting vacuum? Check and see. Pressure sensor plug on?

Swap injectors b/t primary and secondarys and see if the problem is the same, or if the problem now is below 4000rpm. Cost little.

A person can create symptoms similar to what you describe: by removing the vacuum line from the pressure sensor and plugging that vacuum line. Then the pressure sensor sees "0" pressure or the equiv of a LOAD and the secondarys will come on everytime you reach 3800rpm.

Even sitting in the driveway reving the engine. In other words no REAL load required to make the secondarys come online. In the driveway, if you do the above, the engine will reach 3800rpm and ........cycle around that number uless you peg the throttle to the floor. Then it clears up a bit and goes up in rpm.

theory

iTrader: (5)

Pressure sensor plug is on. I have the vacuum line with orifice coming from one of the nipples on the UIM runners beside the injector bleed nipple.

I did exactly what you described, symptoms felt very similar:

In the driveway, I unhooked the vacuum line to the pressure/boost sensor, plugged it, revved it up to 3800 when the LED's came on in the injector clips, and it repeated my symptoms. I did this to confirm my ecu was sending a signal at 3800 to turn on the secondaries.

It felt like this is what happens when I drive the car, which is leading me to believe clogged injector/s since the ecu is sending a signal to the injectors.


I will confirm to make sure there are no issues with the pressure sensor plug/vac line. I didn't know the pressure sensor affected anything at 3800 before. I'll also check to see if I can reach 5-6-7k with very light throttle. If I can do that what do you think the issue is?

Edit: I will check out the pressure sensor GND tomorrow, who knows, maybe that's the culprit.

EmbassyBlack

i have the exact same problem as you.
when i give it tiny bit of throttle it goes up all the way.

HAILERS

The pressure sensor determines LOAD. The secondary injectors come on at 3800rpm only if there is a LOAD. Unplugging the vac line to the pressue sensor and leaving the plastic nipple on the pressure sensor itself open to atmosphere means it's now sensing LOAD.

I meant to say earlier, to remove the pressure sensors vac line and FEEL to see if there is vacuum on that line ...or not. Implying that maybe there is NO vacuum and that is part of the problem.

One or both secondary injectors could be plugged/disabled and you could still reach over 5grand with no hesitation, just by slowly going past 3800rpm. In other words no flooring of the pedal. Just a gradual, gentle acceleration to way above 3800rpm. What does that prove? Damned. I forgot! Means it's not some problem other than a secondary injector problem, I guess. Not thinking very good right at the moment.
TV programm is pissing me off.

theory

iTrader: (5)

Some results.... I have no idea:

I slowly revved the car over 4k and it went up perfectly fine, no hesitation.

Just for kicks I revved it under load, NO HESITATION! WTF?!?! I took it for a test drive and it drove perfectly fine under load or no load, a tiny hesitation at 4k as there was before the S5 swap but nothing nearly the same as before.


My afr's were reading in the 12's above 4k, this makes me think both injectors were working properly (I would imagine they'd be leaner if one wasn't working)

While driving, near the end of the trip the car started dying when stopped, I found a huge vac leak from my intake tube, but there wasn't that leak before so it didn't have anything to do with the hesitation.

HAILERS

If the secondarys wern't working when you hit 3800 rpm under load, the primary injectors duty cycle would be cut in half (normal action), and since the secondarys did not come on line, the car would fall flatt on its face. I mean, you would have cut the fuel amount in half and the engine couldn't help but fall on it's face.

That's how things normally work. Hit 3800 under load.........primarys duty cycle is cut in half...and the secondary injectors come online at the same duty cycle as the primarys.

So you can see how the engine would fall on it's face if the secondarys didn't come online. Once the engine stumbled, the rpms would fall and you'd start a vicious cycle of hesitation/stumble if you held your foot to the floor.

itschrisfoo

do you still have the resistor box in, and are you using s5 injectors

itschrisfoo

if your using s5 injectors you dont need the resistor box. but if your using s4 injectors then you do.

theory

iTrader: (5)

I checked and the pressure sensor vac line was receiving vacuum.

Using S4 NA injectors w/ resistor pack.

Here's my theory: Injectors sat for a long time unused. They were functioning before they were stored. Before install, I didn't apply 12v and listen for a click to make sure they were functioing, I just tested for resistance.

Yesterday was when I applied 12v to check for the click to see if they were working.

I'm thinking maybe I unstuck it when I applied the 12v for testing. I can't think of anything else I did different. It baffles me and kind of scares me as now I'm worried it's intermittent and it will re-appear.

EmbassyBlack

so it works now?

theory

iTrader: (5)

Well the other day it worked. I'm wiring up my VDI/6PI right now, I'll see if it works again today...

theory

iTrader: (5)

Today it worked fine again. Revved beyond 4k under load with no issues. Problem is still unknown but it's working...

6PI and VDI open up as wired.

My only problem is I can't feel any difference from before until now. I always read when the VDI kicks in it feels alot better, but I didn't feel anything happen...

The Shaolin

iTrader: (5)

From dyno graphs I've seen, the VDI is only a few HP until above 7k, where the stock graph drops off and the S5 pulls hard to 8k. The aux ports are really more of a "kick in the pants"

theory

iTrader: (5)

Looks like I'll have to change the rev limiter via RTek to feel the rest!

cptpain

iTrader: (1)

is problem solved or no?

my initial guess was the fuel pump and still is. sound like its showing signs that its on its way out

theory

iTrader: (5)

Well everything is functioning normally. I haven't had any issues in 3 days. Not sure what the problem was or what I did to "fix" it. I will keep a close eye on everything I can check.