Hello all,

I've been reading the boards for a long time and have solved many problems from the info provide here but I seem to have an issue that I just cant solve...

I have a 91 convertible tpi 305 with a 5speed, has shorty headers, (flowtech) true duals with summit 2 chamber mufflers and dumps before the axel.

Car has been sitting for 5-6 years with the odd start here and there, I now have time for her and I want to get this thing running properly.

The car runs and drives great but I have a slight miss at idle, very random misses, I hear faint popping from the exhaust, the motor also shakes a little when idling. If I rev it, I hear a flutter sound, like a paper bag over the exhaust when it revs back down to idle. I can smell the car running rich, especially when coming to a stop.

When driving there is no hesitation or lack of power, car revs up no problem and pulls like crazy. when decelerating in gear I get that flutter sound for low rpm range 1500-800, There are no backfires. when the car is coasting lets say at 50km/h with the clutch in the idle hunts between 800-1200 but when I come to a stop it stays at 850-900... not sure why it does that or if its related???

I have tuned the car up, new cap & rotor, ICM, coil, o2 sensor, temp sensor., I had bosch 3's in it before storage and have replaced them with brand new 3's (didn't solve it).
IAC is good and set (idles 850-900 rpm), TPS is good and set (0.52v). Timing is 6° with wire unplugged.

I have smoke tested for vacuum leaks and I have a small leak at the throttle body shaft... don't think it would be causing my issue

I have new gas, I'm on my 3rd tank... Fuel pressure car off is 42psi and holds, running its 32-33psi, with vacuum capped at FPR its 42, fuel pressure regulator ticks like crazy but seems to be doing its job.

spark plugs and wires look ok, plugs are r45ts, they are old but clean, I took them out and see no fouling.

I have tested each injector wire and they all work, all spark plug wires work.

I am stumped, any ideas what could be causing this?

I have no valve train noise, do you think they need adjusting? As far I know they have never been touched...

ill try to get a sound clip up soon.

Thanks in advance

 

Change the plugs and wires. You may have a high resistance wire, or a cracked plug, etc. You can't always see these defects.

A constant misfire will drive the O2 lean and cause the BLM and INT to add fuel (higher than 128). The rich condition is due to the misfire - you are smelling rich exhaust and unburned fuel.

GD

 

Ok thanks, I will order a new set of plugs and wire and see if that solves it...

 

Just changed plugs and wires and I still have the same issue...

Spark plugs that came out showed no sign of carbon build up, there were all identical.

Do you think my EGR valve might be slightly stick open? I plugged direct vacuum while the engine was running to it and there was not change in idle of roughness...

Any suggestions would be welcomed

 

You might want to do a little reading about the Bosch III injectors; not saying that's your problem, but they are notorious for being not 100% compatible with stock tune.

 

What #/hr Bosch III injectors?

 

They are 19lb injectors, these are my second set on this motor.

would the egr valve give me these symptoms? As mentioned I plugged direct vacuum and no change st idle. Is that normal?

 

Possibly. The EGR should be closed at idle. I suppose it could be stuck open. Have you taken an idle vacuum reading on the engine?

GD

 

Good to know that engine is not "over injected". Many use injectors too large for their engine and wonder why it's rich. And yes as GD said, EGR can have an effect. An EGR valve problem can send the NBo2 sensor voltage high (rich). Need to log to see what's happening. Hard to diagnose problems in the blind.

 

Just took the plenum off and checked the egr valve and it’s clean and working properly.

i really don’t know what can cause a random misfire like this.

 

Any reason why it’s set so high? Do you have a bigger cam? Being that the plugs look ok and it only smells rich at times maybe your car is trying to offset high idle by dumping more fuel occasionally.

 

I set the idle as low as possible with the IAC fully extended and unplugged. Once the min is set that’s the idle I get... I would love to drop it a bit lower but I just won’t go.

 

Good chance this could be your problem or atleast some part of it since it affects fueling and idle plus your original tune would have a lower idle programmed. Check for vacuum leaks. Then try to reset iac.

 

I hooked up the smoke machine and I only gave a small leak at the throttle pivot shaft. No other leaks.

what should the idle be on a 91 manual car?

 

 

^good call! However shouldn’t the idle be set around 650 for tpi unmodded? Anything higher would allow the blades open forcing air in to be compensated for. Thanks for input and correcting anything tuned!

 

You can always set it to 20-25 iac counts. I think if you try to set the minimum idle speed to 650 you will be ok the iac will compensate for the extra target rpm.

 

Again, a log while it's acting up would be helpful.

 

^just realized I thought he set minimum higher. Sorry misunderstood. Yes when warmed up then it would drop assuming the minimum was at 600-650.