No change to RPM when test port B/C jumped
06-01-2023 | 02:30 PM
#1
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 789
Likes: 11
From: Lafayette Louisiana
No change to RPM when test port B/C jumped
So I'm trying to figure out why I can't get my idle set to 840 +/- rpm.
When the car is fully warmed up and no a/c on, the idle is at about at 1000 rpm. Even if I close the mixture screw all the way. This is also true if I jump the B/C ports on the test plug - it affects the RPM only so much before bottoming out and the RPM staying at 1000.
I thought maybe it was a vacuum leak, but I have mostly new silicon hoses and I did a smoke test - no smoke came out from anywhere other than the air filter.
If the engine RPM doesn't change when the test ports are jumped on a warmed-up car would that indicate a bad or stuck ISV? Would a stuck open ISV cause a lean condition under boost or load? the ISV connects post the MAF, so I'm not 100% sure how it would affect engine operation.
When the car is fully warmed up and no a/c on, the idle is at about at 1000 rpm. Even if I close the mixture screw all the way. This is also true if I jump the B/C ports on the test plug - it affects the RPM only so much before bottoming out and the RPM staying at 1000.
I thought maybe it was a vacuum leak, but I have mostly new silicon hoses and I did a smoke test - no smoke came out from anywhere other than the air filter.
If the engine RPM doesn't change when the test ports are jumped on a warmed-up car would that indicate a bad or stuck ISV? Would a stuck open ISV cause a lean condition under boost or load? the ISV connects post the MAF, so I'm not 100% sure how it would affect engine operation.
06-01-2023 | 03:44 PM
#2
Race Director
Joined: Aug 2001
Posts: 10,607
Likes: 49
From: Hudson Valley
I had a similar problem many many years ago and the consensus was there is a small inline fuse somewhere in the circuit. I replaced every fuse and relay in the relay box in the hopes that it may be associated with a circuit that I missed in the diagrams but no luck. I've just dealt with it for the last 15+ years and it is one of the last things to revisit on my "to do" list so I'll be following this.
06-01-2023 | 05:20 PM
#3
Three Wheelin'
Joined: Oct 2021
Posts: 1,363
Likes: 300
From: Southern WI - 89S2 Megasquirt PNP
1) You didnt mention the model/year of your car. Early cars have a binary IACV and later cars have a variable IACV. Is this a 1990-1991 car? Those were the only cars with diagnostic ports I thought? Instead of using the diagnostic port, try unplugging the connector at the IACV instead.
2) Idle RPM is controlled by the amount of air bypassing the throttle plate. IACV more open = more air at idle = higher RPM. I believe there is also an idle screw adjustment on the throttle body for smaller adjustments.
If you have a late car, the IACV is variable which means it opens and closes based on the voltage its fed. When the car is fully warm at idle, its open a smaller amount than when the car is cold at idle (which gives you a higher idle RPM).
Therefore if you believe your idle is off, the first thing I would do is remove the IACV and feed it 12V - does it snap to fully open/fully closed? Is there any binding or slow movement. This would indicate that it needs a thorough cleaning as the gunk and grime (from the AOS oil vapor etc) can build up causing it to not open quite enough, causing a lower idle and an imbalance in air/fuel ratio...
A stuck IACV should not impact your A/F ratio during normal driving or WOT. It only affects the cars idle. All it does is bypass a certain amount of air around the closed (idle) throttle plate to achieve a specific idle rpm. All air, regardless of whether its bypassing through the IACV or not, is being metered by the AFM.
Your lean condition is due to some other factor related to fuel or air metering- FPR, TPS, clogged injectors, clogged fuel filter/tank screen, vacuum leak, AFM issue, DME temp sensor, O2 sensor. Take a fuel pressure reading and go from there. Even better if you can read fuel pressure while under WOT/load to see if its dropping off at higher RPM/load.
2) Idle RPM is controlled by the amount of air bypassing the throttle plate. IACV more open = more air at idle = higher RPM. I believe there is also an idle screw adjustment on the throttle body for smaller adjustments.
If you have a late car, the IACV is variable which means it opens and closes based on the voltage its fed. When the car is fully warm at idle, its open a smaller amount than when the car is cold at idle (which gives you a higher idle RPM).
Therefore if you believe your idle is off, the first thing I would do is remove the IACV and feed it 12V - does it snap to fully open/fully closed? Is there any binding or slow movement. This would indicate that it needs a thorough cleaning as the gunk and grime (from the AOS oil vapor etc) can build up causing it to not open quite enough, causing a lower idle and an imbalance in air/fuel ratio...
A stuck IACV should not impact your A/F ratio during normal driving or WOT. It only affects the cars idle. All it does is bypass a certain amount of air around the closed (idle) throttle plate to achieve a specific idle rpm. All air, regardless of whether its bypassing through the IACV or not, is being metered by the AFM.
Your lean condition is due to some other factor related to fuel or air metering- FPR, TPS, clogged injectors, clogged fuel filter/tank screen, vacuum leak, AFM issue, DME temp sensor, O2 sensor. Take a fuel pressure reading and go from there. Even better if you can read fuel pressure while under WOT/load to see if its dropping off at higher RPM/load.
Last edited by walfreyydo; 06-01-2023 at 05:43 PM.
06-01-2023 | 05:55 PM
#5
Track Day
Joined: Jul 2021
Posts: 18
Likes: 1
From: Toronto
@walfreyydo quick question re IACV, my 86 NA has a really rough idle / vibration at start up however if I hold revs above 1k engine is smooth, the rough idle disappears once car is really warmed up. Do you suspect mine could be stuck / clogged ? Is there a simple way to access / remove without taking manifold off car? Appreciate your insight & expertise. Thanks
06-01-2023 | 06:14 PM
#6
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 789
Likes: 11
From: Lafayette Louisiana
thanks,
I'm trying to parse my questions in a way that isn't too confusing for everybody.
This problem is overlaid on a bigger issue. My car was stock and ran fine. I just installed the Vitesse V-MAF+ conversion and seems to work well, but I must of done something wrong somewhere, though I've back-traced all my steps and all seems correctly installed. I've been talking to John and I'll be getting his MoMonitor system since I already have his chip in my DME. this should speed up diagnosis. I know all my hoses are good, and I do make boost and RPM if I lead the throttle and not smash it to WOT.
These are all the items that popped up after installing the kit:
1. Idle speed issue s mentioned in my original post
2. There is the bucking/cut-out of the engine. Every since installing the Vitesse MAF/Injectors/MAP/Chip, when I floor the car it bucks and almost cuts out. to be clear, It's only when I smash the accelerator to WOT does the car buck/cut out. It's only momentarily that it bucks/cuts out but still continues to run. I cant seem to figure out why. It's like the all of a sudden there is no fuel or at least not enough to have combustion. I have a new fuel damper and 3.0 bar fuel regulator. The damper is brand new, and the regulator has less than 2000 miles on it. I haven't check the fuel filter yet, but honestly, I'm not sure that's the culprit since when I go WOT the bucking/power cut is almost instant. I cant see it how there isn't some residual fuel in the rail that would at least keep the engine trying to accelerate before it bucked. Also the Vitesse kit comes with new injectors, so those are good.
3. If I am able to run the car, lead the throttle and get the engine speed up to 4K+ RPM, and I hold the clutch in and let the RPM fall, it dies sometimes, meaning the RPM struggles to pick back up when it falls under idle speed in time to keep the engine running. I can simply restart the car however.
these are all the pieced i've replaced in the last 2000 miles and all worked before the kit install
ICV
TPS
Fuel Damper
Fuel regulator
Rebuilt Throttle body
Replaced most vacuum hoses with Silicon
Intake manifold gasket
that's a quick list.
I do want to be clear that I don't necessarily think it's the Vitesse kit itself since working with John it appears to be working correctly. He's been very helpful.
I'm trying to parse my questions in a way that isn't too confusing for everybody.
This problem is overlaid on a bigger issue. My car was stock and ran fine. I just installed the Vitesse V-MAF+ conversion and seems to work well, but I must of done something wrong somewhere, though I've back-traced all my steps and all seems correctly installed. I've been talking to John and I'll be getting his MoMonitor system since I already have his chip in my DME. this should speed up diagnosis. I know all my hoses are good, and I do make boost and RPM if I lead the throttle and not smash it to WOT.
These are all the items that popped up after installing the kit:
1. Idle speed issue s mentioned in my original post
2. There is the bucking/cut-out of the engine. Every since installing the Vitesse MAF/Injectors/MAP/Chip, when I floor the car it bucks and almost cuts out. to be clear, It's only when I smash the accelerator to WOT does the car buck/cut out. It's only momentarily that it bucks/cuts out but still continues to run. I cant seem to figure out why. It's like the all of a sudden there is no fuel or at least not enough to have combustion. I have a new fuel damper and 3.0 bar fuel regulator. The damper is brand new, and the regulator has less than 2000 miles on it. I haven't check the fuel filter yet, but honestly, I'm not sure that's the culprit since when I go WOT the bucking/power cut is almost instant. I cant see it how there isn't some residual fuel in the rail that would at least keep the engine trying to accelerate before it bucked. Also the Vitesse kit comes with new injectors, so those are good.
3. If I am able to run the car, lead the throttle and get the engine speed up to 4K+ RPM, and I hold the clutch in and let the RPM fall, it dies sometimes, meaning the RPM struggles to pick back up when it falls under idle speed in time to keep the engine running. I can simply restart the car however.
these are all the pieced i've replaced in the last 2000 miles and all worked before the kit install
ICV
TPS
Fuel Damper
Fuel regulator
Rebuilt Throttle body
Replaced most vacuum hoses with Silicon
Intake manifold gasket
that's a quick list.
I do want to be clear that I don't necessarily think it's the Vitesse kit itself since working with John it appears to be working correctly. He's been very helpful.
Last edited by Last Lemming; 06-01-2023 at 06:16 PM.
