1990 MY Hesitation/ stumble of engine - ZR-1 Net Registry Forums

jrd1990zr1 · 05-16-2021

Was having this problem late last year, When at idle cold there seems to be a miss, like a bad wire. When the engine warms up the situation is not quite as bad but if you step on it the engine hesitates then seems to catch.

Over the winter I pulled the plenum, new set of GM wires, checked the coils. This Spring, same problem. Changed the injectors (FIC) about 9 years ago so they were supposed to be ethanol compliant and at the same time changed the plugs. This was about 30,000 miles ago. Car has 62,000 on it I don't drive it a lot each year.

I am at a loss, seems like next step is another set of plugs?

Can a set of FIC injectors go bad at about 30,000 miles?

Any suggestions on how to diagnose this issue? Is there a simple procedure to check the injectors without pulling the plenum?

Any suggestions would be appreciated. Thanks.

wfot · 05-18-2021

I would check over some simple stuff. check/test the TPS, it might be worn out or in need of adjustment...

Are the plug wires fully seated on each plug?

Are the wires routed exactly like the factory manual Shows?

ZR1 specialist has a detailed way it resistance test the injectors in the car - but the problem is gets worse when the engine is hot.

jrd1990zr1 · 05-18-2021

Quote:

Originally Posted by wfot

I would check over some simple stuff. check/test the TPS, it might be worn out or in need of adjustment...

Are the plug wires fully seated on each plug?

Are the wires routed exactly like the factory manual Shows?

ZR1 specialist has a detailed way it resistance test the injectors in the car - but the problem is gets worse when the engine is hot.

Thanks for the advice. Wires are properly seated and routed, I have already checked that.

Are you aware of a way to check the TPS or adjust it? I had not thought of that.

wfot · 05-18-2021

I don't have the exact sequence at my fingertips right now, but there are 2 ways
1) back probe the middle? and black wire? and read the voltage. it should be at near .54, I think .54 is the optimal. I have used paperclips to not puncture the wire casings

2) hook up ECM reader and just read the voltage

either way the at rest voltage should be the .54 and the open fully should be around 4.7 to 5.0. the upper number is a little less important. the rest number is where you make adjustments. there are 2 torx screws, loosen and twist until you get the .54v.

also, look at the meter as you rotate the throttle open/closed several times. the numbers should be smooth and linear, if there are spots where the numbers stick or jump, the sensor is worn out. I have found GM parts work best as the fueling is designed around TPS reading and MAP readings...

I will look for the exact back probe wire combo if someone else does not jump in with it sooner

John

Ccmano · 05-18-2021

Do you have a 1990 Factory Service Manual and a Tech1 (or similar) scanner?
H

jrd1990zr1 · 05-18-2021

Quote:

Originally Posted by Ccmano

Do you have a 1990 Factory Service Manual and a Tech1 (or similar) scanner?
H

I have the service manual and a set of Gordons ALDL plugs Is there a way to use the plugs to diagnose the this?

jss06c6 · 05-18-2021

What type of plugs are you running? I'd pull them and check gap and condition. Iridium plugs are particularly susceptible to tip erosion in a wasted spark system like the LT-5... I went through a set of NGK 7iX in 4,000 miles! Gap went from .050 to almost .070 on three of the plugs. Switched back to NGK 4177's (TR6), reduced gap to .045".. problem gone.. (stumble off idle under moderate load).

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jrd1990zr1 · 05-18-2021

Quote:

Originally Posted by jss06c6

What type of plugs are you running? I'd pull them and check gap and condition. Iridium plugs are particularly susceptible to tip erosion in a wasted spark system like the LT-5... I went through a set of NGK 7iX in 4,000 miles! Gap went from .050 to almost .070 on three of the plugs. Switched back to NGK 4177's (TR6), reduced gap to .045".. problem gone.. (stumble off idle under moderate load).

Sent from my SM-G950U using ZR-1 Net Registry mobile app

Yes they are AC Delco, meassured gap of #1 plug at a little over 0.040". The NGK 4177 plug (NGK V-Power Nickel Plug Number TR6 Spark Plug) came up as not fitting the 1990 ZR1 engine at O'Reillys when I looked them up. The Advance website says I need to use a NKG 6953. Am I being told the wrong information by the Advance Auto Parts website?

I think I will change out the plugs. Which one should I go for?

jrd1990zr1 · 05-19-2021

Put a new set of AC Delco in gap at 0,035" Checked MAP hoses all good. On start up it seemed smoother will take it for a ride tomorrow.

Problem solved.

My thanks to all who posted suggestions.

jss06c6 · 05-21-2021

Good to hear. How did your test drive go?

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05-18-2021	#2
wfot Join Date: Jan 2009 Location: New jersey Posts: 156	Re: 1990 MY Hesitation/ stumble of engine I would check over some simple stuff. check/test the TPS, it might be worn out or in need of adjustment... Are the plug wires fully seated on each plug? Are the wires routed exactly like the factory manual Shows? ZR1 specialist has a detailed way it resistance test the injectors in the car - but the problem is gets worse when the engine is hot.

05-18-2021	#4
wfot Join Date: Jan 2009 Location: New jersey Posts: 156	Re: 1990 MY Hesitation/ stumble of engine I don't have the exact sequence at my fingertips right now, but there are 2 ways 1) back probe the middle? and black wire? and read the voltage. it should be at near .54, I think .54 is the optimal. I have used paperclips to not puncture the wire casings 2) hook up ECM reader and just read the voltage either way the at rest voltage should be the .54 and the open fully should be around 4.7 to 5.0. the upper number is a little less important. the rest number is where you make adjustments. there are 2 torx screws, loosen and twist until you get the .54v. also, look at the meter as you rotate the throttle open/closed several times. the numbers should be smooth and linear, if there are spots where the numbers stick or jump, the sensor is worn out. I have found GM parts work best as the fueling is designed around TPS reading and MAP readings... I will look for the exact back probe wire combo if someone else does not jump in with it sooner John

05-18-2021	#5
Ccmano Join Date: Jun 2006 Location: Sparks, NV Posts: 2,852	Re: 1990 MY Hesitation/ stumble of engine Do you have a 1990 Factory Service Manual and a Tech1 (or similar) scanner? H __________________ 90'ZR-1 #1461 Red/Black Ported Intake, Degreed Cams, OBX Headers & Exhaust, Fidanza FW, Secondary Delete, Custom tune, C6 GS Wheels. Sold 08 C6 Z51 Crystal Red Metallic Sold Vararam Intake, Tune, Sold 05 C6 Z51 Red/Black Sold 90'ZR-1 #1723 Black/Gray Sold but not forgotten 91' Z51 L98 White Vert. My First, you always remember your first. Sold.

05-18-2021	#7
jss06c6 Join Date: Aug 2015 Location: Magnolia, Texas Posts: 884	Re: 1990 MY Hesitation/ stumble of engine What type of plugs are you running? I'd pull them and check gap and condition. Iridium plugs are particularly susceptible to tip erosion in a wasted spark system like the LT-5... I went through a set of NGK 7iX in 4,000 miles! Gap went from .050 to almost .070 on three of the plugs. Switched back to NGK 4177's (TR6), reduced gap to .045".. problem gone.. (stumble off idle under moderate load). Sent from my SM-G950U using ZR-1 Net Registry mobile app

05-19-2021	#9
jrd1990zr1 Join Date: Dec 2005 Location: Delanson, NY Posts: 914	Re: 1990 MY Hesitation/ stumble of engine Put a new set of AC Delco in gap at 0,035" Checked MAP hoses all good. On start up it seemed smoother will take it for a ride tomorrow. Problem solved. My thanks to all who posted suggestions. __________________ 1968 convertible Lemans Blue, 1986 coupe Medium Blue NCRS Top Flight (sold), 1990 ZR1 #2529 Red NCRS Regional and National Top Flight (sold), 1990 ZR1 #2124 Charcoal Metallic NCRS Top Flight 2010 & 2018, 2003 Anniversary Red Convertible (sold) Last edited by jrd1990zr1; 05-20-2021 at 02:28 PM.

05-16-2021	#1
jrd1990zr1 Join Date: Dec 2005 Location: Delanson, NY Posts: 914	1990 MY Hesitation/ stumble of engine Was having this problem late last year, When at idle cold there seems to be a miss, like a bad wire. When the engine warms up the situation is not quite as bad but if you step on it the engine hesitates then seems to catch. Over the winter I pulled the plenum, new set of GM wires, checked the coils. This Spring, same problem. Changed the injectors (FIC) about 9 years ago so they were supposed to be ethanol compliant and at the same time changed the plugs. This was about 30,000 miles ago. Car has 62,000 on it I don't drive it a lot each year. I am at a loss, seems like next step is another set of plugs? Can a set of FIC injectors go bad at about 30,000 miles? Any suggestions on how to diagnose this issue? Is there a simple procedure to check the injectors without pulling the plenum? Any suggestions would be appreciated. Thanks. __________________ 1968 convertible Lemans Blue, 1986 coupe Medium Blue NCRS Top Flight (sold), 1990 ZR1 #2529 Red NCRS Regional and National Top Flight (sold), 1990 ZR1 #2124 Charcoal Metallic NCRS Top Flight 2010 & 2018, 2003 Anniversary Red Convertible (sold)

05-21-2021	#10
jss06c6 Join Date: Aug 2015 Location: Magnolia, Texas Posts: 884	Re: 1990 MY Hesitation/ stumble of engine Good to hear. How did your test drive go? Sent from my SM-G950U using ZR-1 Net Registry mobile app