Fast idle after intake mod

Topic: Fast idle after intake mod (Read 9374 times) previous topic - next topic

Fast idle after intake mod

February 05, 2014, 06:25:58 PM

I've started work on my SO engine to eek out a few more HPs before I totally tear into the heads, injectors and cam change. I had an HO uppper intake and 60mm TB so I ported the intake and installed it. The power change was dramatic. I shaved a whole second off of my 0-60 time but...... Now I have an 1100 RPM idle that don't seem to be able to bring down. I reused the IAC I had with the SO intake and that already had the IAC spacer with the set screws pretty much screwed all the way in. I reset the ECU to no avail.

I've had an IAC leak before and this is unlike that. It's not pushing me along in every gear like that problem, just idling too high. As soon as I put it in gear it will drop to 700.

I've read that I can make a gasket that seals up part of the IAC port to help with this but is that the only way it's going to happen?

Thanks in advance for the help.

Fast idle after intake mod

Reply #1 – February 05, 2014, 06:50:51 PM

Quote from: marianadeeps;428422

I shaved a whole second off of my 0-60 time but......

And I have a bridge in NY I'll sell cheap... If you can knock a second off any intake swap, you can make millions in the porting business...

If you don't have a vac leak, probably the TPS voltage is high...

Fast idle after intake mod

Reply #2 – February 05, 2014, 07:07:35 PM

Quote from: TurboCoupe50;428424

And I have a bridge in NY I'll sell cheap... If you can knock a second off any intake swap, you can make millions in the porting business....

Just telling you what I've observed. Not scientific by any means. With my iphone timer in one hand and the steering wheel in the other. Maybe my original intake was clogged or something?

Stock: 11.9 sec
8.8/3.73 rear axle swap: 10.5 sec
Intake swap: 9.5 sec

Does that sound crazy?

I'll take a look at look at the TPS voltage setting. I think I read somewhere around here how to do it.... something about needles.....

Fast idle after intake mod

Reply #3 – February 05, 2014, 08:46:02 PM

Stick a vacuum gauge on it and see what it's reading. It should read in the 18"-20" area at a 700-750rpm idle in park and be steady. Any lower and you've got a vacuum leak. My Mark VII with a completely stock 5.0 HO has 18" of vacuum at a 700rpm idle. If it's lower than that I guaranty you have a vacuum leak. Your modifications shouldn't make it have less than 18" of vacuum at idle. My Thunderbird with the modifications listed in my sig makes a rock steady 17" of vacuum at a 750rpm idle. A high idle + intake change makes me think you've got a vacuum leak.

Fast idle after intake mod

Reply #4 – February 05, 2014, 10:41:55 PM

probe the green wire with your red meter lead and the black meter lead bonded to chassis ground

turn on ign key

should "desire" less than one volt dc,,, .6 to .8vdc

now on the same test, keep your red lead in place ,,,take your black meter lead and probe the black wire..
the voltages should be nearly identical.
using stick pins and gator clips makes this test easy

other than than that, hopefully you simply have cracked one of your small plastic vac lines or worse case its a gasket matting issue or vac leak.

if none,,,

did you say you backed off the phillups idle screw?
if you back it off all the way and it still idles high,, then perhaps you put on a cracked intake and didnt know it.

with the car running,,, spray carb cleaner in all the hard to get to spots..... the idle wiill change when you hit the cracked or broken area.

Fast idle after intake mod

Reply #5 – February 05, 2014, 11:50:22 PM

X2. My bet is vac leak. Probably effecting map sensor. check for leaks then recalibrate the iac by unplugging it, idle the car down till it hardly runs, then plug it.back in.

Fast idle after intake mod

Reply #6 – February 06, 2014, 12:10:50 AM

Thanks everyone. I'll check for vac leaks tomorrow if its not pouring, then check my TPS as described above. The idle stop screw is backed off just past touching. Man, I hope it's not a cracked intake -- it was a lot of work to do the porting/polishing.

Fast idle after intake mod

Reply #7 – February 06, 2014, 12:14:51 AM

Check your EGR plate. If you're running an HO upper but using a SO EGR plate or using the HO plate with the SO gasket, you're probably leaking unmetered exhaust gas into the intake. I had this problem when I started mixing parts on mine, and like a moron, I asked parts for an EGR plate gasket for my car instead of for a 96 Explorer. It took forever to figure out what was happening. I checked all over for a vacuum leak, I was literally driving myself crazy over it. Then I happened to look into the intake with the throttle plate open and saw all the gasket hanging into the inlet. I realized what was happening and wanted to shoot myself in the balls for being so stupid.

Fast idle after intake mod

Reply #8 – February 06, 2014, 09:51:02 AM

Quote

Stock: 11.9 sec
8.8/3.73 rear axle swap: 10.5 sec
Intake swap: 9.5 sec

I know Tom mentioned that shaving the 1 second was near impossible, but with numbers THAT dismal I can see it being possible. I think my old V6 pulled better numbers than that. Definitely better than what you list as stock. Regardless, at least your going in the correct direction.

An intake swap from an SO upper to HO upper (BTW -- the lowers are IDENTICAL) should have ZERO effect on your vacuum so long as you put everything back without creating an actual leak.

Putting a vacuum gauge on will help. Would have been nice to know if there was any issue BEFORE the swap as well.

Check the following link out: It's an animated guide for vacuum diagnostics.
http://sbftech.com/index.php?topic=3020.0

Hook up a gauge and let us know what it's doing. I would also suggest taking the IAC off and giving the inner poppet a thorough cleaning with a penetrating oil.

Just a thought too: If the idle screw is backed all the way off, the ECU may be saying "hmm, we need more air" and opening the IAC to compensate. Couple that with an IAC that needs cleaning or is on it's way out can result in some wonky behavior.
After you clean that IAC, do a base idle reset with the electrical connector for the IAC disconnected.

Fast idle after intake mod

Reply #9 – February 06, 2014, 09:54:02 AM

Quote from: TheFoeYouKnow;428458

Check your EGR plate. If you're running an HO upper but using a SO EGR plate or using the HO plate with the SO gasket, you're probably leaking unmetered exhaust gas into the intake. I had this problem when I started mixing parts on mine, and like a moron, I asked parts for an EGR plate gasket for my car instead of for a 96 Explorer. It took forever to figure out what was happening. I checked all over for a vacuum leak, I was literally driving myself crazy over it. Then I happened to look into the intake with the throttle plate open and saw all the gasket hanging into the inlet. I realized what was happening and wanted to shoot myself in the balls for being so stupid.

Not doing that, but sounds like something I might do. The upper came with the EGR spacer already attached and it matches the 60mm TB and intake. But I'm going to kick myself if it's cracked or the upper to lower intake has leak. It was a pain getting that thing back together.

BTW, does anyone know what the little bar brace does in the back of the upper intake. It's kind of an S shape that bolts from the rear passenger upper intake bolt to a bolt in the back of the engine compartment. It's kind of hard to get to and I haven't put it back yet.

Fast idle after intake mod

Reply #10 – February 06, 2014, 12:39:35 PM

I am going to suggest that it's running in fail safe mode. Is it possible to get codes from it? This is all just a guess.

Fast idle after intake mod

Reply #11 – February 06, 2014, 01:42:41 PM

Quote from: custompunk;428485

I am going to suggest that it's running in fail safe mode. Is it possible to get codes from it? This is all just a guess.

You're saying you think the ECU is stuck in fail safe and it needs to be reset/rebooted somehow? Is this something I can do (capture the codes) or do I need to find some piece of equipment? sorry I'm new to all of this.

Fast idle after intake mod

Reply #12 – February 06, 2014, 03:08:48 PM

Vacuum leak FIND IT!!! Or tps out of adjustment. One second gain in 0-60 with an intake swap.
I have a Bridge in Brooklyn for sale cheap. And just for the record those engines do not have a Screw to set base idle!!

Fast idle after intake mod

Reply #13 – February 06, 2014, 03:22:27 PM

Quote from: marianadeeps;428426

Stock: 11.9 sec
8.8/3.73 rear axle swap: 10.5 sec
Intake swap: 9.5 sec

Jebus I just saw that. The completely stock standard 5.0 in my Thunderbird would hit 60mph in about 8.5 seconds or less after a swap to 3.73 gears.

Fast idle after intake mod

Reply #14 – February 06, 2014, 03:23:40 PM

Quote from: TOM Renzo;428492

Vacuum leak FIND IT!!! Or tps out of adjustment. One second gain in 0-60 with an intake swap.

+1. It has to be one of these two. It's not the computer as you didn't mess with it. An intake swap is not going to throw the computer into failure mode.