Bucking and misfiring at low rpm. Direction needed. - ZR-1 Net Registry Forums

Young1 · 12-10-2019

I am helping someone prepare their car for NCRS performance review judging in February. 1993 9k mile beautiful.
Fresh fuel.
Vacuum system is tight.
Secondaries function normal per pink wire black stripe test.
Idles well. Will idle down the street in 4th gear smoothly!
Revs well.
No D/M flywheel knock.
No check engine or other lights on screen.
For instance when performing the 1st to 4th Skip shift and apply even moderate throttle it will buck and misfire considerably. It can be heard in the exhaust.
Will occur in any gear between 1-3k rpm. Above this runs normal.
A fórum search reveals many different responses.
What do I test or replace next?
O2 sensors?
Primary fuel pump?

Paul Workman · 12-10-2019

FUEL - AIR - SPARK

Nothing like a dynamic scanner to sort out a plethora of parameters: what is or isn't normal.

That said, and assuming you don't have a scanner to do a dynamic scan, and knowing the secondary pump is triggered at different levels of throttle and rpm, depending on MY, you can verify (at least) both fuel pumps are drawing the right amounts of current - indicative of their function. But, also measuring the dynamic fuel pressure (gauge taped to the windshield while test driving) can provide very useful data.

And, eventually or as a matter of being comprehensive, I'd want to verify the fuel system and filter/lines are clean; a topic for another time, perhaps.

Anyway...I'd focus on fuel delivery first; starting with dynamic pressure especially in the low rpm/throttle region where its having the issue.

Young1 · 12-10-2019

OP update. Fuel pressure is 52 and 43. Static tested. Perfect. I think I found the issue. #7 plug wire was routed by GM alongside a bare braided ground strap. There was some carbon discoloration on the original plug wire. Of course it would misfire on #4 as well with the dual plug coil. Rerouted and re insulated. No time yet for test drive. Looking for the simple!

Young1 · 12-13-2019

Original platinum tip spark plugs had .050-.060 gaps. Not good. A set of Delco 41-602 did the trick. Missing resolved.

dredgeguy · 12-13-2019

Quote:

Originally Posted by Young1

Original platinum tip spark plugs had .050-.060 gaps. Not good. A set of Delco 41-602 did the trick. Missing resolved.

Well done Steve.....Congrats!

12-10-2019	#1
Young1 Join Date: Sep 2016 Location: Ocala, FL Posts: 881	Bucking and misfiring at low rpm. Direction needed. I am helping someone prepare their car for NCRS performance review judging in February. 1993 9k mile beautiful. Fresh fuel. Vacuum system is tight. Secondaries function normal per pink wire black stripe test. Idles well. Will idle down the street in 4th gear smoothly! Revs well. No D/M flywheel knock. No check engine or other lights on screen. For instance when performing the 1st to 4th Skip shift and apply even moderate throttle it will buck and misfire considerably. It can be heard in the exhaust. Will occur in any gear between 1-3k rpm. Above this runs normal. A fórum search reveals many different responses. What do I test or replace next? O2 sensors? Primary fuel pump? __________________ Steve Young 1991 ZR-1 Quasar Blue Metallic/Black #729 Second Owner 1991 ZR-1 Dark Red Metallic/Black #2040 SOLD 1990 ZR-1 Dark Red Metallic/Saddle #1670 SOLD Double your happiness.....complain half as much.

12-10-2019	#2
Paul Workman Join Date: Nov 2007 Location: Squires (near Ava MO in the Mark Twain N'tl Forest) - Missouri Posts: 6,493	Re: Bucking and misfiring at low rpm. Direction needed. FUEL - AIR - SPARK Nothing like a dynamic scanner to sort out a plethora of parameters: what is or isn't normal. That said, and assuming you don't have a scanner to do a dynamic scan, and knowing the secondary pump is triggered at different levels of throttle and rpm, depending on MY, you can verify (at least) both fuel pumps are drawing the right amounts of current - indicative of their function. But, also measuring the dynamic fuel pressure (gauge taped to the windshield while test driving) can provide very useful data. And, eventually or as a matter of being comprehensive, I'd want to verify the fuel system and filter/lines are clean; a topic for another time, perhaps. Anyway...I'd focus on fuel delivery first; starting with dynamic pressure especially in the low rpm/throttle region where its having the issue. __________________ Good carz, good food, good friendz = the best of timez! 90 #1202 "FBI" top end ported & relieved Cam timing by "Pete the Greek" Sans secondaries Chip & dyno tuning by Haibeck Automotive SW headers, X-pipe, MF muffs Former Secretary, ZR-1 Net Registry

12-10-2019	#3
Young1 Join Date: Sep 2016 Location: Ocala, FL Posts: 881	Re: Bucking and misfiring at low rpm. Direction needed. OP update. Fuel pressure is 52 and 43. Static tested. Perfect. I think I found the issue. #7 plug wire was routed by GM alongside a bare braided ground strap. There was some carbon discoloration on the original plug wire. Of course it would misfire on #4 as well with the dual plug coil. Rerouted and re insulated. No time yet for test drive. Looking for the simple! __________________ Steve Young 1991 ZR-1 Quasar Blue Metallic/Black #729 Second Owner 1991 ZR-1 Dark Red Metallic/Black #2040 SOLD 1990 ZR-1 Dark Red Metallic/Saddle #1670 SOLD Double your happiness.....complain half as much.

12-13-2019	#4
Young1 Join Date: Sep 2016 Location: Ocala, FL Posts: 881	Re: Bucking and misfiring at low rpm. Direction needed. Original platinum tip spark plugs had .050-.060 gaps. Not good. A set of Delco 41-602 did the trick. Missing resolved. __________________ Steve Young 1991 ZR-1 Quasar Blue Metallic/Black #729 Second Owner 1991 ZR-1 Dark Red Metallic/Black #2040 SOLD 1990 ZR-1 Dark Red Metallic/Saddle #1670 SOLD Double your happiness.....complain half as much.