Missing @ Idle - Pros only pls...
Thread Starter
Racer
Joined: Oct 2003
Posts: 273
Likes: 0
From: Midland Texas
Missing @ Idle - Pros only pls...
I am considered a professional mechanic. I bought an 88 coupe, built a new motor. Installed a mild cam, otherwise stock L98. Pulls very hard, but I can feel a miss at idle and only at idle. Not a hard miss, just suttle, but eratic, and consistant at the same time. Definately not cam lope. Anyway, I have replaced of couse the plugs and wires. Then the pickup and module. I rigged a setup in the shop and cleaned the injectors on the bench with B-12 and moitored the spray pattern, looked excellent to me. I replaced the EGR and eliminated all possibilities of vacuum leaks. My base timing is at 6 degrees in drive and shows 42 degrees on the scanner maxed out after reconnecting the shunt. Replaced ECM also, but not the prom. I guess I need to get off my lazy --- and run the tests on the coil. One last thing - Sometimes the car fires immediately, especially when cold. Have to turn it over twice to fire when warm generally. Coils show failure when warmer. Hope to cure bot problems in one shot. What do you think???
Team Owner
Joined: Nov 1999
Posts: 26,679
Likes: 17
From: This vette goes to 11 Milford MA
St. Jude Donor '03-'05
Re: Missing @ Idle - Pros only pls... (autotech)
You may put off some people with that title. I am NOT a professional mechanic, neither are most of the guys here. I would bet my left nut that these guys know more than any "professional" about L98's. :thumbs:
That being said,:D what I have seen more than once is a pinched or bad ground, especially if its a Super Ram equipped car since they block the view of most of the top end. The other issue to check is the injectors. If the car is cold and fires instantly but cranks when hot, it could be the injectors. When the factory injectors get old, they have resistance issues when hot. Usually the hard start is followed by a puff od smoke from the cylinders washing down. When I swapped in some FMS injectors into my 88, the miss and hard hot starts went away completely.
That being said,:D what I have seen more than once is a pinched or bad ground, especially if its a Super Ram equipped car since they block the view of most of the top end. The other issue to check is the injectors. If the car is cold and fires instantly but cranks when hot, it could be the injectors. When the factory injectors get old, they have resistance issues when hot. Usually the hard start is followed by a puff od smoke from the cylinders washing down. When I swapped in some FMS injectors into my 88, the miss and hard hot starts went away completely.
Thread Starter
Racer
Joined: Oct 2003
Posts: 273
Likes: 0
From: Midland Texas
Re: Missing @ Idle - Pros only pls... (Fastguy)
I opologize to the forum as you are probably correct about the knowlege herein. It could be entirely possible that I have a resistance breakdown in the injectors as I have listened to them with a scope, actually today and some are louder than others, but did not notice a pulse differentiation. Thank you for your input... :rolleyes:
Melting Slicks
Joined: Sep 2000
Posts: 3,099
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From: PITTSBURGH PA
Cruise-In IV Veteran
Cruise-In V Veteran
Re: (scorp508)
if you have a miss which hole is it in you can narrow it down a couple of ways you first let the motor sit till its cold to the touch then fire up the engine for just a minite or so and touch the header tubes very quickly to see which hole is dead then you can narrow it down from there do this test then get back to us i know it sounds neanderthal but it works when you do not have a infra red temp gun the tubes should be scalding hot but one will be just warm that is the dead hole if you have a miss this test will find it :cheers:
Team Owner
Joined: Nov 1999
Posts: 26,679
Likes: 17
From: This vette goes to 11 Milford MA
St. Jude Donor '03-'05
Re: Missing @ Idle - Pros only pls... (autotech)
The GM factory injectors are expensive so most people have used the Ford Motorsports. You can usually get a set cheap in the for sale forum or on ebay.Good Luck.
Team Owner
Joined: Sep 2001
Posts: 62,160
Likes: 1,733
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
Re: Missing @ Idle - Pros only pls... (autotech)
Hard startups when warm usually indicate injector failure of some sort.
Sometimes you can pull plugs and check.
Does it surge at idle when warm at all?
Sometimes you can pull plugs and check.
Does it surge at idle when warm at all?
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Team Owner
Joined: Sep 2001
Posts: 62,160
Likes: 1,733
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
Re: Missing @ Idle - Pros only pls... (autotech)
Try hooking up a fuel pressure gauge at the rail, shut the car off and see how long pressure holds, it should hold for a good while.
If it drops, one or more injectors is stuck open.
If it drops, one or more injectors is stuck open.
Thread Starter
Racer
Joined: Oct 2003
Posts: 273
Likes: 0
From: Midland Texas
Re: Missing @ Idle - Pros only pls... (autotech)
I think they may have hit on it above. Just because I a saw a good spray pattern on the bench does not mean the injectors are functioning properly after warmup, I do believe the mixture is improper at idle, even though the 02 sensor is switching from rich to lean as it should...I think the 02's are managing it, therefore no code...
Instructor
Joined: Sep 2002
Posts: 116
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From: Boise ID
Re: Missing @ Idle - Pros only pls... (autotech)
This might be a long shot but I would verify that the voltage at the fuel pump is the same as the voltage at the battery. I had a mysterious miss as well as a few other problems with my 88, I went through all the standard stuff and in the end it was a simple corroded connection in the fuel tank. When I was investigating the fuel pressure was good on a gauge but when I pulled the return line and watched the fuel flow there was a slight pulse to it. Cleaned the connector and it ran like a champ.
Good luck
Good luck
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,009
From: Texas
Re: Missing @ Idle - Pros only pls... (autotech)
I think they may have hit on it above. Just because I a saw a good spray pattern on the bench does not mean the injectors are functioning properly after warmup, I do believe the mixture is improper at idle, even though the 02 sensor is switching from rich to lean as it should...I think the 02's are managing it, therefore no code...
Check the resistance of the injector coils. Should be ~15 ohms, +/- 4 ohms. Be sure to check them hot (after engine warmed up) & cold (after sitting overnight). Sometimes when the injectors are bad, the symptoms typically appear and are worse after the ECM goes into closed loop...just depends on what side the bad injector(s) is/are on.
[Modified by 96GS#007, 6:41 AM 11/6/2003]
Drifting
Joined: Feb 2003
Posts: 1,978
Likes: 2
From: San Antonio Texas
Re: Missing @ Idle - Pros only pls... (autotech)
This may not be anything relating to your engine, but I've noted a slight idle miss after changing from Bosch plugs, to Nitto. This is backwards from what is expected, but that's what I've got. I'll try something different next time.
I used to be a mechanic.
:chevy
I used to be a mechanic.
:chevy
Thread Starter
Racer
Joined: Oct 2003
Posts: 273
Likes: 0
From: Midland Texas
Re: Missing @ Idle - Pros only pls... (96GS#007)
That all makes good sense. What about these FMS Injectors? Where do you get them? How much? and which ones for my 88? Appriciate the input you guys. Lots of help... :thumbs:
Melting Slicks
Joined: Apr 2002
Posts: 2,118
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From: St. Charles, MO Route 66 Corvette Club
Re: Missing @ Idle - Pros only pls... (autotech)
Autotech: Look at your injectors! If they are grey in color with a silver band about in the middle, I may have the solution to your problem! Give me your comments!