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Poor idle ONLY when driving

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Old 06-19-2017, 11:08 AM   #1
JasBass
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Default Poor idle ONLY when driving

Good morning everyone,

Total newbie to the forum and this is my first post. Thank you in advance for taking the time to read through this and offer some suggestions, tips and tricks. This is my first corvette purchase (total rescue car) and I picked up an 89 L98 6 speed. I was given a 91 FSM and have been using the L98 specific info in there to get me to this point. Car details and story available on my profile.

Car had originally broken down twice and subsequently went through and replaced distributor cap, rotor, wires, plugs, injectors, MAF and coolant temp sensors. I'll spare the details as to how I got there, but it was a month of FSM reading, multimeters and a few too many 6 packs.

Car currently takes about 2-3 seconds to fire and will fire strong and idle just fine in the driveway from open to closed loop. Runs wonderful through the rev range, no miss or backfires. Just normal crackles coming down off revs. Idles great at 800, occasionally to 900 (has a mild over stock cam and bored 30 over, cam specs unavailable). Even revving in neutral (6 speed), coming down off revs, settles right on 800. And that's both cool and hot engine temps. But when I take the car out and begin to actually drive the vehicle, I get some incredibly minor bucking between 1-2k rpm. When slowing down (car in gear with clutch depressed) the car does not hold that beautiful idle it holds in the driveway, will slip down to 500-600 rpm but will respond to throttle and it perks right up. Runs like a champ on the road otherwise Pull back in the driveway, throw it in neutral and she goes back to the great 800 idle and revs full and happy.

I throw no warning lights (other than my ABS which I am addressing later). And I have a cooling fan running all the time which I'm hunting down next (suspect bad wiring or relay). My gut is telling me it's a tune issue since I had to reflash the PROM when upgrading injectors (22-32lb done by wonderful crew PCM for less). PCM had to guess on tune figures since I was unable to track down a build sheet from previous owner or shop but I'd say they got it pretty damn close. Fresh fuel filter. I wanted to throw this out there to make sure there wasn't something bone headed I was missing since this is my first Vette.

Thank you for your time! I'm excited to get her restored and looking good again.

Last edited by JasBass; 06-19-2017 at 11:14 AM.
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Old 06-19-2017, 12:19 PM   #2
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Welcome to the forum and thanks for the details.

My quick guess is that you have fuel pressure issues, which are very common. Your car idles and starts well, which requires the LEAST amount of pressure. But you said that once you get to about 2k it starts bucking, then settles down once in neutral. New fuel injectors like higher pressure.

Get yourself a Fuel Pressure Gauge, pop it on the Schrader Valve, tape it to your window and take it for a ride. I'll be a dollar to a donuts that your pressure needle twerks more than a drunk sorority girl after some heavy day drinking.

Then get a new pump to compliment your filter. You will be happy again. Also spring for a 89 manual as the 91 and 89 have some significant differences.

Well done and congrats for joining our crazy.

Last edited by billschroeder5842; 06-19-2017 at 12:23 PM.
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Old 06-19-2017, 02:42 PM   #3
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Originally Posted by billschroeder5842 View Post
Welcome to the forum and thanks for the details.

My quick guess is that you have fuel pressure issues, which are very common. Your car idles and starts well, which requires the LEAST amount of pressure. But you said that once you get to about 2k it starts bucking, then settles down once in neutral. New fuel injectors like higher pressure.

Get yourself a Fuel Pressure Gauge, pop it on the Schrader Valve, tape it to your window and take it for a ride. I'll be a dollar to a donuts that your pressure needle twerks more than a drunk sorority girl after some heavy day drinking.

Then get a new pump to compliment your filter. You will be happy again. Also spring for a 89 manual as the 91 and 89 have some significant differences.

Well done and congrats for joining our crazy.

Thanks for the welcomes! It's definitely been a journey so far.

I was also leaning towards a pump issue but was starting to rule it out when the engine revved so well. In neutral, it will cleanly rev from idle to 5,500 without a hiccup. But actually drive the car and that little baby, almost unnoticeable buck from 1,000-2,000 shows up, will rev past that cleanly, but then doesn't want to come nicely back to idle and ends up around 500-600 until I feather the gas. I'll get a gague to check and log pressures of neutral revs vs. driving pulls. I have a new stainless sending unit rig to install anyway so a pump swap would be perfect timing. I've considered going to an adjustable pressure regulator to set up for future mods. Pretty sure it's the original pump. At this point, the lack of maintenance and proper part fitting to compliment the 383 doesn't surprise me. This car was abused and never maintained...
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Old 06-19-2017, 03:54 PM   #4
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New fuel injectors like higher pressure.
I don't follow. Why would they like or dislike higher or lower pressure? They are rated for 43.5 psi and whatever the poundage so how does that help?
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Old 06-19-2017, 04:02 PM   #5
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I don't follow. Why would they like or dislike higher or lower pressure?
I wasn't being technical I was being glib.
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Old 06-19-2017, 04:36 PM   #6
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I wasn't being technical I was being glib.
I thought you were trying to say that it works better with higher pressure which was why I asked. You weren't, were you?
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Old 06-19-2017, 05:42 PM   #7
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I thought you were trying to say that it works better with higher pressure which was why I asked. You weren't, were you?
If you want to get technical, whatever pound injectors you have will atomize fuel better with increased pressure. Hence why a lot of people will run lower pound injectors at higher pressure. They won't "work" better. I do believe his point was about atomization not work efficiency. At least that's how I read his comments
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Old 06-19-2017, 06:33 PM   #8
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If you want to get technical, whatever pound injectors you have will atomize fuel better with increased pressure. Hence why a lot of people will run lower pound injectors at higher pressure. They won't "work" better. I do believe his point was about atomization not work efficiency. At least that's how I read his comments
That's what I thought you were talking about. What I don't know is how true it is. Back in the early 90s, I got an AFPR. Cranking up to about 10 psi above stock with 2 psi increments did not yield any better mpg and if it did increase power, it was probably imagined. I'd gas up and cruise 100 miles there and back for a job every week.
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Old 06-19-2017, 07:01 PM   #9
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Does the speedometer work or is the cluster out of the car? My car ('86) ran like that after the 4+3 to T56 swap until I got the speedometer reconnected and reading properly.
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Old 06-19-2017, 07:39 PM   #10
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Does the speedometer work or is the cluster out of the car? My car ('86) ran like that after the 4+3 to T56 swap until I got the speedometer reconnected and reading properly.
Interesting. Speedo and Tach work but the polarizing film on the whole unit is garbage. I can read it only when I wear polarized sunglasses. But who complains about cruising in sunglasses during summer in their Vette lol. Fuel level doesn't show at all and neither does the oil temp. The rest of the divisions work, oil pressure, range, avg mpg, volts, etc. I believe oil temp has some input to the ECM, but not confident how involved it is in fuel mapping and delivery. I've got a new sending unit rig to install, old one is rusting around the filler, so swapping in a new pump is probably worthwhile for now and checking those pressures. Even though its a fresh fuel filter, throwing another one in during the unit/pump install may not be a bad idea. I'll dig down some more on cluster input, thanks for the idea
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Old 06-19-2017, 07:41 PM   #11
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That's what I thought you were talking about. What I don't know is how true it is. Back in the early 90s, I got an AFPR. Cranking up to about 10 psi above stock with 2 psi increments did not yield any better mpg and if it did increase power, it was probably imagined. I'd gas up and cruise 100 miles there and back for a job every week.
lol agreed. The only real benefit I see is getting a better atomized spray in the intake and a cleaner burn
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Old 06-19-2017, 08:44 PM   #12
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lol agreed. The only real benefit I see is getting a better atomized spray in the intake and a cleaner burn
My thought is while it is great for the treehuggers or greenie weenies, unless I can see some tangible benefit, it would have to be for a race team where 1 thousandth of a second could be the difference between winning the prize purse or going home.
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Old 06-19-2017, 08:58 PM   #13
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My thought is while it is great for the treehuggers or greenie weenies, unless I can see some tangible benefit, it would have to be for a race team where 1 thousandth of a second could be the difference between winning the prize purse or going home.
Yessir. Your thoughts leaning more fuel pressure issues, maybe some IAC work or spark advance/load tables in the tune? I'll pick up a FP test gauge tomorrow. Still building up the tool set since buying the car.


Benny- Getting readouts on the cluster but the polarizing film is junk and can only see with polarizing sunglasses. Only thing not registering in the cluster is oil temp and fuel level. New sending unit rig going in and will throw a pump in there as i'm sure its the original pump or close to original...
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Old 06-19-2017, 10:08 PM   #14
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Yessir. Your thoughts leaning more fuel pressure issues, maybe some IAC work or spark advance/load tables in the tune? I'll pick up a FP test gauge tomorrow. Still building up the tool set since buying the car.


Benny- Getting readouts on the cluster but the polarizing film is junk and can only see with polarizing sunglasses. Only thing not registering in the cluster is oil temp and fuel level. New sending unit rig going in and will throw a pump in there as i'm sure its the original pump or close to original...
Ok, it looks like you have that covered. Good luck with it.
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Old 06-20-2017, 07:33 PM   #15
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Quick update:

Haven't had time to pick up a FP gauge, but I was doing some quick tinkering/cleaning under the hood. Pulled the vac line off of the FPR and did smell a little gas, no wetness or overwhelming fumes, but definitely a smell. Starting to feel like this is working up to an FPR and pump issue. Will update
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Old 06-20-2017, 07:59 PM   #16
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Quick update:

Haven't had time to pick up a FP gauge, but I was doing some quick tinkering/cleaning under the hood. Pulled the vac line off of the FPR and did smell a little gas, no wetness or overwhelming fumes, but definitely a smell. Starting to feel like this is working up to an FPR and pump issue. Will update
New FPR. Not sure if it is worth paying for adjustable. I wouldn't.
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Old 06-21-2017, 08:26 PM   #17
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New FPR. Not sure if it is worth paying for adjustable. I wouldn't.
Agreed. Lemmie bounce something off of you. Ever heard of the FPR diaphragm/spring getting temporarily stuck (creating higher pressure state) and then after a short period of time return to normal? Parts store is a decent drive so I haven't been able to pick up a FP gauge yet with after-work obligations, but was tinkering more under the hood tonight. Starts up a little slow, still within normal range, warms up and idles nice. revs idle to red clean. But then I did some mid range revs, about 2,500-3,000 and one of the times it finally started to run crappy, rich smoke coming out, choke-y idle, tapped the gas a coupe more times and it evened out. Really suspecting FPR issue at this point
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Old 06-21-2017, 10:57 PM   #18
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Agreed. Lemmie bounce something off of you. Ever heard of the FPR diaphragm/spring getting temporarily stuck (creating higher pressure state) and then after a short period of time return to normal? Parts store is a decent drive so I haven't been able to pick up a FP gauge yet with after-work obligations, but was tinkering more under the hood tonight. Starts up a little slow, still within normal range, warms up and idles nice. revs idle to red clean. But then I did some mid range revs, about 2,500-3,000 and one of the times it finally started to run crappy, rich smoke coming out, choke-y idle, tapped the gas a coupe more times and it evened out. Really suspecting FPR issue at this point
I never heard of a spring getting temporarily stuck. But lets say it did, how does that explain the gas smell if there is no leak in the diaphragm?
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Old 06-22-2017, 06:49 AM   #19
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I never heard of a spring getting temporarily stuck. But lets say it did, how does that explain the gas smell if there is no leak in the diaphragm?
Case and point, it doesn't. Just trying to account for it running perfect the majority time and then it slipping into the rich/stumbly condition occasionally. All evidence says FPR at this point, but sometimes I push too hard for a definitive root cause... New diaphragm is on the way
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Old 06-22-2017, 08:12 AM   #20
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Case and point, it doesn't. Just trying to account for it running perfect the majority time and then it slipping into the rich/stumbly condition occasionally. All evidence says FPR at this point, but sometimes I push too hard for a definitive root cause... New diaphragm is on the way
Fuel leaks out from the small tear in the diaphragm and goes into the manifold and you get a rich condition. If the ECM can adjust, you don't see it. If not, well....
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