Engine Idle issue 91' GT with vortec supercharger

fordmanTN

New Member
Jun 24, 2021
2
0
1
47
TN
I have a 91' GT with a Vortec VR1 supercharger, aluminum heads, 70 mm eldebrock throttle body, upper and lower intake, 80 lb injectors and a few other things. This car has been giving my tuner nightmares, at first it wouldn't idle without surging and dying. He got it where it will idle when it's warm nut it still surges on a cold start until about 110 degrees then it seems to idle fine after that. It has a new IPS, ICV and PCV valve and screen. I have sprayed ether all around the motor, enough to make me nervous and it never changes, so it seems no vacuum leaks any more. HELP!!!!!!!
Most of the time when it acts up after it warms up a bit and starts surging it seems that I can lightly pump the brake pedal and it will settle out. At one point I unhooked the vacuum line from the booster thinking their might be another leak but nothing changed. This thing is driving us both crazy! The tuner is very well known and has tuned 100's of cars but this one seems to be pain in the butt!! I will not give out his name because I am just here trying to find out opinions of other folks that may have had the same issue.
 
  • Sponsors(?)


jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,461
2,794
234
75
Dublin GA
lowendmac.com
I have a 91' GT with a Vortec VR1 supercharger, aluminum heads, 70 mm eldebrock throttle body, upper and lower intake, 80 lb injectors and a few other things. This car has been giving my tuner nightmares, at first it wouldn't idle without surging and dying. He got it where it will idle when it's warm nut it still surges on a cold start until about 110 degrees then it seems to idle fine after that. It has a new IPS, ICV and PCV valve and screen. I have sprayed ether all around the motor, enough to make me nervous and it never changes, so it seems no vacuum leaks any more. HELP!!!!!!!
Most of the time when it acts up after it warms up a bit and starts surging it seems that I can lightly pump the brake pedal and it will settle out. At one point I unhooked the vacuum line from the booster thinking their might be another leak but nothing changed. This thing is driving us both crazy! The tuner is very well known and has tuned 100's of cars but this one seems to be pain in the butt!! I will not give out his name because I am just here trying to find out opinions of other folks that may have had the same issue.
You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
 

ThinBlue502

Advanced Member
May 7, 2019
229
386
73
39
Ohio
You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
I tried unsuccessfully for a very long time to fix my idle issue. I found that thread and fixed it the same weekend. Use it. Love it. Enjoy your surge-free idle.
 

fordmanTN

New Member
Jun 24, 2021
2
0
1
47
TN

I'd start by seeing what your vacuum reading is at idle
According to my boost/vacuum gauge it's around 12 at an idle. Does that sound right? I found a few small leaks when we first started tuning the car so went all around checking for leaks and think I have them all.
I saw the other post about dumping the codes but don't think that will help since mine has a quarter horse chip.
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,461
2,794
234
75
Dublin GA
lowendmac.com
According to my boost/vacuum gauge it's around 12 at an idle. Does that sound right? I found a few small leaks when we first started tuning the car so went all around checking for leaks and think I have them all.
I saw the other post about dumping the codes but don't think that will help since mine has a quarter horse chip.
That seems a bit low with a stock camshaft.

Finding vacuum leaks

Revised 6 May 2018 to add carbon canister plumbing as a common leak area.

There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and every hose and connection.

Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-725 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as starting fluid, propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

After you have done the simple visual checks and the check for vacuum leak on the underside of the intake manifold, consider doing a smoke test.
Some of the guys here have built smoke machines used to find automotive vacuum leaks. They seem to work quite well and are made mostly with parts you would have laying around in your garage. Check out smoke machine vacuum leak - YouTube and see if there is one that you could build.

The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

The PCV grommet and the power brake booster check valve grommet are two places that often get overlooked when checking for vacuum leaks. The rubber grommets get hard and lose their ability to seal properly. The PVC grommet is difficult to see if it is correctly seated and fitting snugly.

The hoses and connections for the evaporative emissions (carbon canister and purge valve) are other common sources of vacuum leaks. The large vacuum outlets on the bottom side of the upper intake manifold are common hiding places for deteriorated vacuum lines and caps over unused vacuum ports.

Fuel injector O rings can get old and hard. When they do, they are prone to leaking once the engine warms up. This can be difficult to troubleshoot, since it is almost impossible to get to the injectors to squirt oil into the fuel injector mounting bosses. If the plastic caps on the fuel injectors (pintle caps) are missing, the O rings will slide off the injectors and fall into the intake manifold.

Fuel injector seal kits with 2 O rings and a pintle cap (Borg-Warner P/N 274081) are available at Pep Boys auto parts. Cost is about $3-$4 per kit. The following are listed at the Borg-Warner site ( http://www.borg-warner.com ) as being resellers of Borg-Warner parts:
http://www.partsplus.com/ or http://www.autovalue.com/ or http://www.pepboys.com/ or http://www.federatedautoparts.com/

Most of the links above have store locators for find a store in your area.

Use motor oil on the O rings when you re-assemble them & everything will slide into place. The gasoline will wash away any excess oil that gets in the wrong places and it will burn up in the combustion chamber. Heat the pintle caps in boiling water to soften them to make them easier to install.



Diagram courtesy of Tmoss & Stang&2birds
mustangFoxFordVacuumDiagram.jpg



Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.

photodisplay.jpg


Determining if you have a leak due to a slipped intake gasket as shown above. This test is only good if you can get the engine to run somewhere in the 1000-1700 RPM range
If your valve cover oil filler & PVC systems are still in the original configuration, try this:
Cap or plug the hose from the intake manifold to the PVC valve with a bolt.
Cap or plug the PVC valve with a piece of hose with a plug or bolt in it.
At that point the only vent for the crankcase is the tube from the oil filler neck to the throttle body.

Disconnect the tube that runs from the oil filler neck to the throttle body. Make sure the oil filler cap is on securely. Start the engine and put your thumb over the end of the tube that comes from the oil filler cap. If you feel suction, there is a leak. Another thing to do is to extend the tubing from the filler neck so that there is enough to stick the end in a jar or cup filled with motor oil. If it sucks up the oil, you definitely have a leak at the underside of intake manifold.

This isn't necessarily the definitive test, but it is the best thing I could come up with on short notice. If there is a lot of blowby, this obviously won't be of much help.

See the picture below to see the breather tube where in connects to the throttle body. It is close to the TPS and runs over the top of the IAC.

TPS_IAB_Pic.jpg




I built a smoke generator to test for leaks with parts from Home Depot. It wasn't expensive, and found a leak in the EGR valve that I didn't expect.
Google https://www.google.com/search?q=diy...gBAKABAaoBB2d3cy13aXrIAQjAAQE&sclient=gws-wiz .



The following are diagrams courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring

Fuel, alternator, A/C and ignition wiring

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

Vacuum diagram 89-93 Mustangs

HVAC vacuum diagram

TFI module differences & pinout

Fuse box layout
 
Last edited:
  • Useful
Reactions: 1 user