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Engine stumbling or missing

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Old 10-23-2018, 03:40 PM
  #1  
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Join Date: Oct 2001
Location: North Carolina
Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
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?
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Old 10-23-2018, 04:04 PM
  #2  
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Location: northern VA
Posts: 1,317
Car: 88 Sport Coupe Camaro
Engine: V6 2.8
Transmission: Borg-Warner T-5
Axle/Gears: RPO/GU6: 3.42
Re: Engine stumbling or missing

What scan tool are you using?

Your best chance of diagnosing drivability issues is to view the same thing the ECU is seeing at the time the event occurs.
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Old 10-23-2018, 04:25 PM
  #3  
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Join Date: Mar 2017
Location: Portland, OR
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Car: 86 Imponte Ruiner 450 GT
Engine: Various
Transmission: 700R4 3000 stall.
Axle/Gears: 9 Bolt Torsen 3.70
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
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Old 10-24-2018, 10:49 AM
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Join Date: Oct 2001
Location: North Carolina
Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
Re: Engine stumbling or missing

Originally Posted by W.E.G. View Post
What scan tool are you using?

Your best chance of diagnosing drivability issues is to view the same thing the ECU is seeing at the time the event occurs.
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.
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Old 10-24-2018, 10:55 AM
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Join Date: Oct 2001
Location: North Carolina
Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
Re: Engine stumbling or missing

Originally Posted by GeneralDisorder View Post
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
That could be my problem.
Thanks for the advice.
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Old 10-24-2018, 11:02 AM
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Join Date: Oct 2001
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Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
Re: Engine stumbling or missing

Originally Posted by 305TBI View Post
That could be my problem.
Thanks for the advice.
By the way, the Block Learn is a stable 128 and the Integrator is a stable 127.
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Old 10-24-2018, 11:41 AM
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Posts: 1,569
Car: 86 Imponte Ruiner 450 GT
Engine: Various
Transmission: 700R4 3000 stall.
Axle/Gears: 9 Bolt Torsen 3.70
Re: Engine stumbling or missing

Originally Posted by 305TBI View Post
By the way, the Block Learn is a stable 128 and the Integrator is a stable 127.
Is that in closed loop? At idle or cruise or both?

GD
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Old 10-24-2018, 01:21 PM
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Join Date: Oct 2001
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Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
Re: Engine stumbling or missing

Originally Posted by GeneralDisorder View Post
Is that in closed loop? At idle or cruise or both?

GD
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.
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Old 10-24-2018, 02:58 PM
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Car: 86 Imponte Ruiner 450 GT
Engine: Various
Transmission: 700R4 3000 stall.
Axle/Gears: 9 Bolt Torsen 3.70
Re: Engine stumbling or missing

When was the last full tune up?
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Old 10-25-2018, 02:06 PM
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Join Date: Oct 2001
Location: North Carolina
Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
Re: Engine stumbling or missing

Originally Posted by GeneralDisorder View Post
When was the last full tune up?
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.
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Old 11-10-2018, 02:17 PM
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Join Date: Oct 2001
Location: North Carolina
Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
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.
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Old 11-10-2018, 05:28 PM
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Join Date: Mar 2017
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Posts: 1,569
Car: 86 Imponte Ruiner 450 GT
Engine: Various
Transmission: 700R4 3000 stall.
Axle/Gears: 9 Bolt Torsen 3.70
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
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Old 11-11-2018, 10:42 AM
  #13  
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Join Date: Oct 2001
Location: North Carolina
Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
Re: Engine stumbling or missing

Originally Posted by GeneralDisorder View Post
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
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.
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Old 12-05-2018, 06:15 PM
  #14  
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Thread Starter
 
Join Date: Oct 2001
Location: North Carolina
Posts: 100
Car: 1991 Firebird Formula
Engine: 305 TBI
Transmission: 700R4
Axle/Gears: 4.10
Re: Engine stumbling or missing

Originally Posted by GeneralDisorder View Post
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
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
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Old 12-06-2018, 11:58 AM
  #15  
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Join Date: Mar 2017
Location: Portland, OR
Posts: 1,569
Car: 86 Imponte Ruiner 450 GT
Engine: Various
Transmission: 700R4 3000 stall.
Axle/Gears: 9 Bolt Torsen 3.70
Re: Engine stumbling or missing

Excellent! Thank you for the resolution.
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