Got all vacuum leaks fixed and my stock 87 Turbo is idling fine at 750 rpm but there is hesitation/missing when accelerating at full throttle and low rpm. Fine at high rpm (3000+) or normal acceleration. TPS is spot on. Ideas on what's the most likely cause? Thanks in advance.

 

Perhaps it is time to replace the fuel sock and fuel filter.

Have you looked into rewiring your fuel pump?

 

Already did that. Also, no problem at high rpm, so don't think fuel pressure is problem.

 

Locate the Fuel Resistor Box under the air box and disconnect the plug. Then jumper one of the Blue/Red wires to one of the Blue/Green wires in the same plug and leave the plug disconnected after the jumper is in place. This bypasses the unit and you'll be able to tell whether this is the root cause for your problem. If not then remove the jumper and then replug to the box.

 

didn't seem to help. By the way, it's a blue wire, not blue/green that needs to be jumpered to bypass the resistors. But, after reading more posts, I think I am running into the fuel cut above 7.5 psi boost that's programed into the ecu. Either need to keep a light foot on the accelerator or get a fuel cut defender I guess. Thanks for the help.

 

And as far as the TPS is concerned have you checked it for dead spots as well besides having it set to a specific reading as stated in the FSM? And I apologize for the incorrect wire identification as I looked at the S5 and not S4 diagram.

 

So how are you hitting fuel cut at low RPMs and not high RPMs?

If you were actually hitting fuel cut, it wouldn't really matter what the engine speed was, right? As long as you were making "X" amount of boost pressure...

 

Both good points. Plus, doesn't the jumper test eliminate the fuel cut ability of the ecu. That's why you had me check that right? Wouldn't fuel cut cause more of a loss of power, rather than an actual miss. The missing seems to occur about 3200-3300 rpm, with boost. I'll check the tps for dead spots next.

 

Why? The jumpering that you did bypasses an item which decides how much voltage is sent to the fuel pump and nothing more. Under load the unit sends 12 volts to the pump while under steady cruising the unit sends about 9 volts to the pump. And it's entirely possible that poorly conditioned wiring to the fuel pump could create some of the same issues. Your TPS w/engine completely warmed should range from 1 volt at idle to almost 5 volts at full throttle or actually around 3000 rpm which is when the TPS plunger is fully released. After that occurs the very same signal is sent to the ECU. And an analog voltmeter will help you identify dead spots better than a digital one as it is too sensitive.

 

tps is perfect. smooth increase from 1 to 5 volts.

 

Based on a very recent experience, let me jump in here.

Ever since the new engine was put in my S4 TurboVert I've had this same hesitation. Drove me nuts as nothing seemed to work including having the injectors sent out, back flushed, and re-tuned.

Add, ever since I purchased my S4 Vert in 1991 I've had a rough idle problem during summer months while running the AC causing my engine temps to move up the scale. Once engine temps dropped, idle is fine.

Add, recently I developed this same rough idle and normal engine temps. So I started looking for a vacuum leak. Everything hooked up; I decided it was time to bring my friend Robert (FC3S91) into the picture what with his greater mechanic's mind. He started spraying injector cleaner all around the engine, and every time he shot the spray under the UIM toward my injectors, idle would smooth out. We guessed the problem was I had used rounded edge S5 injector lower grommet's instead of the squared edge S4 versions.

So this weekend I pulled the injectors to replace the lower grommets and while I was at it I pulled the diffusers from the block to check for issues. What I found was that every O-Ring was hard as a rock which caught me by surprise as everything was supposed to be new on the block when I bought it. I also noted that the holes in one of the secondary diffusers was clogged with carbon. So since that also could be a cause of a vacuum leak due to expansion when the block heats up, I replaced all the O-Rings (thank you Robert). I also re-opened up the holes in the secondary diffuser by cleaning the carbon out of the holes.

Result:

1. Rock solid idle at start up.

2. Rock solid idle while running the AC even after the load of the compressor had caused the engine temps to rise up.

3. NO HESITATION between 3000-4000 RPM. None, Nada, Niet, zero.

Add to this, every thread I've read about this hesitation suggests the root cause of the dreaded 3000-4000 RPM hesitation is related to the transitioning of the 2ndary injectors kicking in. Thus, all proposed cures was electrical (primarily improve electrical grounding to the engine block). This experience suggests the cure to this infamous hesitation problem isn't just electrical.

Moral of the story.

1. You haven't fixed every vacuum leak if you haven't replaced the injector grommets and diffuser O-Rings.

2. Add to the list of cures for the 3000-4000 hesitation the replacement of the lower grommets and diffuser O-Rings. Also make sure the diffusers aren't clogged up.

 

Wow man great info. Thanks for sharing.

 

Great information. Now I need to decide if it's something I can live with or do I want to tear it down and replace grommets and o-rings.

 

Thanks to Demon721 for pointing out that timing can cause this hesitation issue. Turns out, sure enough, my timing was off. Fixed that and voila!, no hesitation issues.

 

you just cured all of my headaches, i was having the same problem, just took a quick glance at my injector o rings and the cracked and hard. will replace them asap and go from there.

 

Double check your trailing coils are going to the correct plugs.

 

Glad rotoman got your car running good.
I have the same problem, it hesitates or misfires between 1200-3k rpm. Any higher and its fine, it happened after I removed the emissions junk.
Idles fine and starts right up at 1200rpms, and I just re-timed it, TPS is accurate, and I cleaned and flowed my injectors.
Any ideas where I should be looking at?

 

If the trailing wires were mixed you would know something was up right away.it Makes the Car real tough to drive..Runs like ***..
But,at the same,It's a good habit to get into though.Check plug wires to eliminate the "obvious".

 

The idle speed needs to be under 1000 rpm to set timing correctly. Otherwise the ecu advances the timing. If you can't get to idle under 1000, you probably have a vacuum leak or items on the throttle body are not adjusted properly. Most likely a result of the emissions work done.

 

+1 to the leaky injector gromets and seating. I had great acceleration through the RPMs except the lower band; in addition to, I lost the engine completely and nearly the car due to a fire (Since the injectors came loose and started spraying on top of the block and onto the Turbo).

 

Replaced grommets, o rings on injectors and diffusers. I put new spark wires and plugs on too. I would rev the engine up and while it decelerates it pulls a vacuum of 21-22 I believe this tells me I don't have a vacuum leak?
I will put the top end back on monday or tuesday cause I got a new clutch slave and its easier with my stuff off to install it.
I have a stock ecu and rtek 2.1 and either or makes the hesitation/misfire. But I will write down what my rtek says in diagnosis and post here to see if it checks out with you guys because I am not the best with tuning/diagnostics.
I also have my bac still hooked up, it works because it "clicks" when I apply power and negative to it, but when I connect the harness it does nothing, I can plug and unplug and it will not do anything. I'm sure if I can get a tip on where to check for the power source for that I can get the idle lower hopefully in order to do the timing correctly.
Much appreciate the help guys.

 

When unplugging the BAC the engine should not change much at all if the car is not under load. Try unplugging the BAC w/the A/C on or the heater on full along w/the headlights on. When unplugged, the engine should bog and want to stall. The ECU controls the ground signal to the BAC which tells the item how much it should work.

 

Ok, I'll keep it plugged in no matter what then. Just didn't know if I was supposed to hear a clicking at all when it was on. But the screw on the side I will mess with to get my idle below 1k rpm.

 

Fixed*

It was my tps, had dead spots. Got a used one now its all smooth, spent alot of money where it didn't need to go but hopefully this helps people with the same problem.

Unplug the TPS, if hesitating or drops rpm while accelerating, and while the tps is unplugged, rev the engine. If revs quickly, then get new tps, this was my problem. I set the tps but I didn't try unplugging it and reving the engine.

 

I ve seen a similar problem. S5 T2 Primary injectors weren't seated properly (actually they were pinched between fuel rail and block) They were sitting on the pintle cap instead properly sitting in their spot. Fixed that and sure enough it runs a lot better, no missfiring at high rpms and better idle. Idle is still not rock solid but drivable. I suppose thats due to another vacuum leak resulting from a bent secondary fuel rail. One douche took the engine out by hanging the lift on the fuel rail. *double facepalm*
I am pretty sure if we replaced the fuel rail and put in some new o rings that thing would run like a champ.