Engine stumbling or missing
 

I am the original owner of a 1991 firebird 305 TBI with a SN-92 Paxton. The supercharger was recently rebuilt by Paradise Wheels.
While driving the car at any steady cruising speed (30 mph to 70 mph) there is a random miss or stumble every few seconds.
I just rebuilt the TBI completely.
The idle is reasonable.
The car accelerates very good with no hesitation.
No trouble codes found with a scan tool.
Any ideas what to look for?

Re: Engine stumbling or missing
 

Re: Engine stumbling or missing
 

You have a lean cruise situation most likely. Have to check out the BLM's, etc. Could be a bad O2 sensor. The narrow band systems only use the O2 for idle and cruise. Throw a calibrated wideband up the tailpipe and check the idle/cuise O2 readings against the wideband. If there's more than half a point of AFR difference between the two under idle or cruise then you most likely have a bad sensor.

GD

Re: Engine stumbling or missing
 

AutoXray XP240. It plugs into the OBD1/2 connector and scans the same data that the ECU sees.
I use the capture mode to store the data and then review the data later.
I thought all scan tools did this.

Re: Engine stumbling or missing
 

That could be my problem.
Thanks for the advice.

Re: Engine stumbling or missing
 

By the way, the Block Learn is a stable 128 and the Integrator is a stable 127.

Re: Engine stumbling or missing
 

Is that in closed loop? At idle or cruise or both?

GD

Re: Engine stumbling or missing
 

That is closed loop. I have not recorded data at idle; the car idles ok. I have recorded data 4 different times .. One at a steady constant 40mph, then 50mph, then 60mph and 70mph. All scan tool data is similar. The rich/lean data flips as expected, The oxygen sensor goes well beyond the mid point and below the mid point.

Re: Engine stumbling or missing
 

When was the last full tune up?

Re: Engine stumbling or missing
 

I take care of my car on the inside and out on a regular basis. I purchased it on 7-16-1990 (it is a 1991 model). However, I don't change the spark plugs as often as I would like. The car is modified and changing the plugs is a REAL BEAR.
I might add that today I was driving my car and I got a code 13, oxygen sensor fault.
The block learn and integrator both were at 131 and the oxygen sensor read between 330 and 409.
I will have to get the proper sensor and replace the original one (it is 28 years old).
I will keep you and the board posted.
Thanks to all who have helped and are interested in this post.

Re: Engine stumbling or missing
 

Update -
I replaced the oxygen sensor (it was faulty) and I adjusted the "minimum air" hoping that this might correct the engine missing problem, but that had no effect. As I mentioned in an earlier post the car runs great under acceleration and deceleration, but at constant steady speed the missing is quite noticeable. I pulled the number 3 spark plug and found it to be carbon fouled. The remaining plugs I suspect are in the same condition. I think that the plugs have maybe 10,000 miles on them. If I have the plugs replaced I will have to have it serviced because I am too old to do that now (81 years). Still no trouble codes set. Any additional help would be very much appreciated.

Re: Engine stumbling or missing
 

Sounds like maybe it's ready for an ignition tune up or at least clean and gap the plugs. It's not unusual to need this more frequently on modified forced induction engines.

Is it wet, oily carbon, or dry powdery carbon on the plugs?

GD

Re: Engine stumbling or missing
 

Thanks GD.
It's dry.
I also replaced the distributor cap. The spring loaded center pin inside was not functioning smoothly. I thought that was the problem, but not the case.
I will have the plugs replaced.

Re: Engine stumbling or missing
 

Problem Solved-
I had the spark plugs replaced and discovered that the #5 plug had a particle shorting the electrodes.

Thanks for all your help and interest.
305TBI

Re: Engine stumbling or missing
 

Excellent! Thank you for the resolution.