Hi all, I just had a new low backpressure exhaust put on my 86 NA. Now my motor idles really bad when its cold and stalls out if I take my foot off the accelerator pedal. After fully warmed it idles around 800. I figure something needs adjustd but I just don't know what.

SO..... what should I adjust.

thanks.......... ash

 

It sounds like the usual cold idle problem when the fast idle cam is removed (did you do the TB mod?) - that's pretty much not related to your new exhaust though. I've heard that the cam can get stuck, though it usually gets stuck in the high-idle position, not the low-idle position. I think I'd check that and the water thermo sensor. Since it idles well after it's warmed up, that narrows things a bit!

 

I think it has to be related to the exhaust replacement since it idled perfectly before. I noticed that the exhaust has more of a gasoline smell to it now. Like maybe its running too rich. Would going to a lower back pressure exhasut system make the motor run rich? Could the poor cold idle be caused by the rich mixture? If it is running too rich, how do I correct that?

ash

 

You can adjust the idle AFR with the variable resistor (S4 only); check it out here. That will also bring your idle to the recommended 750 instead of the 800 you get now. To make sure the baseline's good, recallibrate your TPS too. It's unusual for an exhaust system to change idle that much, and my guess is something got knocked or unplugged when the exhaust was swapped. Check for obviously unplugged connectors, like the ACV (above the exhaust manifold, pics here), and for vacuum leaks. If you haven’t done so already, it will help if you tap into your diagnostics plug; that might point you towards something like your water thermo sensor.

How much of your exhaust system was replaced? Do you still have catalytic converters? Did you install a header?

 

Man, those word ads make it an obstacle course to pick out the links I put in there!

 

I just had the stock mufflers replaced with a straight through design. Would the decrease in backpressure cause any problems?

 

Shouldn't - all it could do is delay the auxiliary ports' opening time, but if you still have cats, that shouldn't be a problem.

 

Maybe it was just a coincidence because now it stalls out every time I take my foot off the gas pedal. Cold or hot, doesn't matter. Even when trying to maintain a 25 mph cruise, I can hear the motor missing. Sort of a sputtering type sound. It accelerates great. Lots of power.

ash

 

I’ve been there before. I was driving my sister around once when a hose on the intake funnel popped off (I just rerouted the BAC inlet). I had to keep my foot down to keep it from stalling, and ever since then she’s convinced my car’s a ticking time bomb...

Definitely check for vacuum leaks, and check the AFM too per FSM "fuel and emissions" page 4-54. If possible get the baseline healthy by recalibrating the TPS, idle speed, and idle AFR, and by checking the diagnostics plug; I provided links for those above. Good luck!

 

Is your car running rich? That could be why you smell gas in the exhaust, usually that happens when putting an aftermarket exhaust on. You need to lean it out with the adjustor under the hood. As for the stalling in cold weather during startup, has it been sitting all night? I have the same problem when the cold cames around (45 degrees or so). I store mine in the winter, so it doesnt bother me to much.

 

Actually a low backpressure unit will decrease the vaccuum the exhaust makes for pulling the exhaust gasses out of the chamber quickly. On a piston driven car if you have to low of an exhaust backpressure itl actually cause a decrease in Horsepower and TQ. In this case it might be the same thing. All id do is check the TB Cables and make sure theyre not lax, and if so tighten them up abit. Also adjust the normal idle to something abit quicker, like 100 RPM quicker. If that dosent solve it, you got me.

 

I think Rocket's really talking about exhaust velocity, not backpressure, but let’s please not debate exhaust physics here, especially when only the mufflers were replaced (i.e. the cats are still there). Let’s just see if we can cure this 7’s idle issues!

The car stalls unless the gas pedal is down; it's not holding idle at all, warm or cold. That usually means a big vacuum leak or AFM problems. I'd first look for vacuum leaks and check the AFM, then adjust the idle speed and idle AFR here (Make sure you jump the set connector and warm up the car first):
http://howto.globalvicinity.com/gv_w...i=61&co=1&vi=1

Tap into the diagnostics plug here:
http://howto.globalvicinity.com/gv_w...i=39&co=1&vi=1

Recalibrate the TPS here:
http://howto.globalvicinity.com/gv_w...i=58&co=1&vi=1

 

Wow. its unbeleiveable the amount of information you guys have. I really appreciate your help.

I'm at school right now, so I'll probably check those things tonight.

But, I was thinking.......... (dangerous , right)... When the new mufflers were installed, they didn't use clamps, they were mig welded in place.

Could that have caused any problems? Like maybe frying my O2 sensor. Or Not.

ash

 

No problem, it's cool how these forum communities break down information barriers. When in past history could one anonymous person casually ask the world of enthusiasts a tech question? It's also cool how ready this community is to help each other.

The diagnostics plug should tell you if the O2 sensor is working - keep in mind it has to be up to operating temperature before it works correctly (the diagnostics plug might tell you the O2 sensor is broken until it's up to temp). If the welders clamped their power supply to the chassis instead of the exhaust pipe, I suppose there's a chance the O2 sensor was part of the welding circuit (i.e. bad). Let's hope that's not the case, because the ECU is another part of that circuit! If the ECU still works, when the O2 sensor's malfunctioning, the ECU uses a fail safe by taking the O2 sensor out of its calculations, and I don't believe that would result in the lack of idle you're seeing. Give the steps I suggested above a try and let us know how it goes.