This is on an 87 base NA. I used a Fluke 189 multimeter for all the tests.

Mods done to the car (to originally help solve this problem, but to no avail): No cats (straight-through 2" piping to the stock muffler (single passenger-side). Provides ample back-pressure to operate the 5/6pt actuators, which open at 3152rpm at WOT (tested by hooking a mini-toggle switch to them). Removed air pump, blocked-off ACV, blocked-off EGR, no vacuum rack (except for the "crank-case" vent. No Pulsation Damper. No cold-start system. Engine has not been proted. Stock exhaust manifold.

Alright, this is a problem that has been annoying the hell out of me for the past 3 months now. Here's everything I've done to try and solve the problem:

At first I thought it was a problem with the fuel system. I bought a brand-new fuel filter (Mazda OEM). I pressure tested and flow tested it. All was good, but to help avert any future problems, I did a direct battery-rewire to it. I used the original power wire (ran through an anti-flood switch) to activate the switch of the relay (12v 40a SPST solid state). Voltage increased from 11.735v to 14.223v DC. Flow wasn't affected, but it did raise the pressure by ~10 psi. Next, I moved on to the injectors. I tested them by removing the fuel rails, zip-tie'ing them to the bosses and having a friend crank the engine. All 4 (tested 2 at a time on the primary rail) sprayed a nice even pattern at cranking-speeds.

Next target was all the sensors. At an ambient temperature of 68d celcius, everything tested out perfectly. I didn't check the air-temp sensor (on the dynamic chamber) at anything above or below ambient temperature, and the same applies to the water thermo-sensor (on the water pump housing). These are going to be my next involved test. The TPS is working flawlessly, and all the ignition coils. I haven't tested the AFM, but I did replace it with a known good unit from my (formerly mine anyways) 88 GTU. New spark plug wires and plugs. Timing is set correctly, and the hall sensors (inside the CAS) are functioning perfectly. How do you test the boost sensor with a vacuum pump and a multimeter (which leads should be connected?) and what readings should it give me? When I tested it, all I got were a bunch of solid numbers that didn't fluctuate as vacuum increased/decreased. I tested it disconnected from the car.

So, from here I moved on to grounding issues. I re-gounded the ECU (2C, 2R, 3A and 3G) to no avail, as well as re-soldered every joint inside the ECU. The ONLY abnormality I saw was that 2 of the blue resistors inside the ECU were a little brown, but (removed from the PCB) their resistance was still within 5% of a new pair (which I installed). Added several more gounding wires per the writeup under the hood (engine, coils, boost sensor, ect..)

Now, the problem is exacerbated when it is cold outside. If it's a hot summer day, the problem is ALMOST eliminated. When it's a brisk day, it's very prominant. AFAIK, the Boost sensor's internal diaphragm, AIT sensor (both; inside the MAF and on the dynamic chamber), and water thermo sensors should be the only things affected by this...

So, what am I missing? This is driving (no puhn intended) me insane! I don't have a known good S4 (86-87.5) ECU to swap out, but I'll be searching for one shortly if nothing else smothes out 5k-6k hesitation.

Aside from all that stuff, the engine is in fair mechanical condition (3 even bounces of ~90psi on the front and 3 even bounces of ~85psi on the rear). I have not yet removed the double-throtte system. The car idles VERY rough (at 750rpm) when hot. It will flood every time it is between 110d-150d fahrenheight (engine temp). Anything colder or hotter and it'll start right up. Sometimes my BACV doesn't want to work, nor does my AWS solenoid (AWS sometimes fail at cold startup, BACV sometimes fails at hot startup). I've done many, many TPS calibrations/idle mixture adjustments and none solved the rough-idle.

I did notice that the injectors are fluctuating in duty cycle at the time the hesitation occurs.... They're dropping duty cycle load, though the exact numbers I forget.

Anyhow, I'll quit rambling and let you guys pick at this one!

 

Ever check for vac leaks?

 

Three things I would be looking at from your description...

#1 The missing PD. If you replace what ever you have blocked the PD off with; a real PD, does it fix the problem. Fuel rail issues would be worse if the fuel and rail were cold.

#2 Vacuum leak

#3 if you block off the air input to both the BAC and the AWS (just plug the line- or pinch the line), does that solve the problem. Both can leak and are on when the engine is cold.

 

I had a problem kinda like that. My car was a n/a s5 car. Had a exhaust, intake, etc. Would run fine under light throttle, but under wot, the car would hesistate around 5k, then again around 7k. I did everything I could think of. Fuel pump, injectors, cas, plugs, wires, reground everything, afm, ecu....but nothing would help. I ended up taking it to a dyno, and found out under wot, it would go pig rich. I MEAN PIG RICH! The wideband would only read to 10-1, and there where places where it was even richer than that. I ended up putting a s-afc to lean the car out. I ended up putting down nearly 170rwhp on a mustang dyno once everything was right. Never did figure out what was wrong with it. I ended up trading the car in on my Rx-8. Of course I got rid of it with the stock exhaust, intake, no s-afc, etc....once all the bolt-ons where removed, the car seemed to run fine. Kinda weird. CJ

 

I've checked for vacuum leaks, and I even replaced all 6 hoses (FPR, 3 for the PCV, 1 for the hose that runs from the UIM to the LIM, and the boost sensor hose with the restrictor pellet installed) with silicone hoses. The only ones I didn't replace were for the oil metering system (5 hoses); I'll check those today.

The PD was a non-issue; I tried 2 different ones, and finally just eliminated it. I'm not going to rule it out though...

I'll try capping-off the BAC/AWS system after I replace all the remaining hoses for the oil injectors.

I'll hook my multimeter up to the O2 sensor to see what voltages it's giving me around 5k-6k. Just as a reference, when I tested it with a blow-torch, the peak value I got was something around 1.721v.

Thanks a lot guys! If there's any other input, it's all appreciated!

 

yeah ive heard other people talk about how modded s5s tend to go rich especialy under WOT. mine doesnt stumble but i know its running well beyond stoich. i gonna get an safc when i fing the time and money, get the most of my mods.

 

Okay, here's what I found out today:

I replaced all the vacuum hoses. The 5 for the oil injectors, removed the PCV and went with an open-air filter (oil-catch type), and removed the double-throttle system. I re-did the gasket on the EGR block-off plate and also, I noticed that the air-bleeds on the secondaries were clogged with crud, so I just removed them. The hesitation is way better now! It still has a very slight (can't even feel it unless you're really looking for it) hesitation right at 4870rpm, but nothing meriting a problem (yet). Oh, and it was around 20d fahrenheight outside today, and it has never ran well when it's cold outside before.

As best as I can tell, the main problem with the hesitation was due to the clogged air-bleeds. Since they were clogged, they were "dripping" fuel instead of atomizing it. But that doesn't explain why it was temperature dependant. hmmm....

As far as the idle quality is concerned, it's a WORLD better now. Somewhere between the removed PVC and the double throttle mechanism, it got fixed. Sweet.

I didn't bother hooking a multimeter to the O2 sensor since it's running well now. I didn't replace the PD either.

Thanks for your help guys!

 

dp

 

tp