Car is a 1991 Formula w/ a 305 TPI and a T5 tranny.

Problems:

• Runs smooth in open loop but stutters at low RPM cruise when closed
• When engine speed slowly increases, it will all of a sudden lose power and jerk around, especially in 1st gear
• Closing the throttle in lower gears will cause it to buck and quickly decelerate, and you can hear it go "whomp whomp" from the exhaust

Replaced:

• Newer plugs and wires
• New distributor w/ module (eBay special)
• New ignition coil
• New fuel pump
• Rebuilt Design III 19# injectors
• Coolant temp sensor
• Relatively new O2 sensor

I've done my very best to troubleshoot this problem, which has persisted in some form or another for over 4 years. Timing is set at 6 degrees BTDC. Fuel pressure w/ vac removed is 42, vac on at idle is around 34 PSI. Tried driving it with vac unplugged to see if low fuel pressure was the issue, no dice. I have also driven it with the fuel pressure gauge attached, and it showed nothing unusual. I've slightly tightened the gap on the plugs, they themselves don't have a nice tan colour but certainly don't look bad. Holds good vacuum of around 18.5 inches Hg at idle, and only moves slightly. Engine ticks but all valves on O2 sensor side are working.

TunerPro RT tells me that my TPS is perfect, and my MAP and MAT are consistent. Spark advance is from 17 to 22 degrees at idle, and increases to about 48 at 80 MPH. Knocks a few times on hard acceleration. What I find most interesting is that the Integrator will drop right off when it stumbles hard, down to 90 at times. Block learn is consistenly below 128 while cruising, and is 160 at idle. I need help interpreting these numbers because I'm new to this.

Sorry for the length, but all I can think of is either bad ECM or PROM (its a custom) or a crappy O2 sensor (NGK 2 years old).

 

Just went out and ran a few more tests. All injectors ohmed out between 18.4 and 19.1 hot. When you slowly open the throttle from idle, the revs will die down and will not hold steady. For example, if I rev it to 1300, it will fall back to 900, and then start to bounce around steadly somewhere in between. Then if you close the throttle completely it will drop to 400 then back up to 700 where it usually idles. And it doesn't sound healthy!

 

Anything? Maybe that 50 dollar distributor is part of the problem?

 

I'm having the about the same issue. One thing I noticed is the timing. just off idle the timing will drop about 5 to 8 degrees btdc and then pickup and will increase as normal. I'm thinking the knock sensor or ing module. Will check tomorrow.

 

hmmm... almost sounds like a bad map sensor.

 

What tells you it is the map sensor and how would you check it? "Thanks for the quick reply.

 

did you find out if it was the map sensor?

 

Darnit! I too am having a very similar problem. I dropped a 383 stroker and when I reach 2,000 RPS -- drops to about 1,000 and chokes when I give her gas. The worst part about it when at idle speeds...it feels as though she will shut down.

Any ideas?