Engine ‘86 gt/ Auto. Low vacuum/ idle surge/ no leaks


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May 19, 2021
Hey Y’all,
Never had an F.i Mustang. Just picked up an 86 GT vert.
88k miles. stock everything Took it home, installed headers back exhaust w/ cats
70mm tb(I know too big probably)
Removed smog pump and sealed vacuum lines and holes in heads

after the work was done, she was idling rough. Surging like crazy. Smelled rich af( burning my !)

pulled the codes 21,31,42,92
Then did a
Smoke Test:
EGR valve was leaking
Removed the EGR and capped the vacuum lines, blocked off opening

replaced ECT (code 21)

still surging, I decided to smoke test again ( I’m smoke testing theough the vacuum tree?)
No leaks
-pulled the pcv, plugged the hole and the line( check intake leak) no suction at filler breather
-tested fuel pressure, all good

Vacuum test***12in at idle, consta

Like a noob, I just figured, Imma throw an IAC at this thing and do a reset
—this actually worked for about 25minutes then it started surging again

ran the scanner again. 31(egr gone), 42,92 richy rich. Did the cylinder test also. All are good
Replaced the 02 sensors, no change

still surging
I’ve been over timing, tps voltage, several smoke tests, idle resets

the car drives amazing, the idle is just completely screwed. I mean it really cruises nicely, just when let off she’s pulsing and not being nice

This car actually has a smaller 55 series battery in it. The battery bled acid all over my new driveway a few weeks ago right before I worked on it. Hasn’t happened since. Will probably replace with a proper size tomorrow
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act sensor and ect sensors is what id go with along with making sure plugs are gapped right and good, fuel pump and filter are good and of course making sure your tining is right by getting a piston stop and seeing if your balancer is marked right.. mine wasnt
I have an ‘86 also, and with a new IAC, I found that I had to switch some wires on the plug. It’s been a while, but I will try to find what I did and post back here. After I did this, it helped my idle issues.
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Put the stock Throttle body back on the car. Also, pick up and install a new EGR, they're not expensive.

You will gain exactly zero horsepower with that intake and set of heads behind a 70mm throttle body.

You just shifted the bottleneck to the intake manifold on a fuel-injected car that operates by Manifold Absolute Pressure. :O_o: Consider this while you're reinstalling the OEM throttle body. :)
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Here is the thread I found that explained the “switching the wires” on the newer IAC’s. It worked for me.

Here is the thread I found that explained the “switching the wires” on the newer IAC’s. It worked for me.
Thanks for this. Gave it a try last night. not much difference as I'm still suffering from extremely low vacuum with no leaks. Not even intake. Going to change the plugs and replace every vacuum hose. I'm hearing about Vacuum ports being clogged. Then maybe a stock TB (I threw mine out SMDH)
act sensor and ect sensors is what id go with along with making sure plugs are gapped right and good, fuel pump and filter are good and of course making sure your tining is right by getting a piston stop and seeing if your balancer is marked right.. mine wasnt
I tested the ECT and it ohm'd correctly with the ambient temp. Really looking into the timing. I advanced the timing last night basically as far as I could turn the dizzy and it started pulling 15" vacuum (I couldn't detect ping-or maybe I don't know what it sounds like :/)Im going to recheck TDC 2 more times today. I've checked for intake leaks several times (Plugging pcv lines and checking suction at filler to TB hose, none found) installed some new plug because it has been extremely rich. Still extremely low vacuum (8-10in) at idle
Ok, Rechecked timing and TDC, Set to 14deg. pulls 8-10in of vacuum(raises as lowers as it surges/chugs)

pulled the intake hose and covered the TB. Motor died right away ruling out other air leaks in the upper intake right?

I guess the next step in pulling the intake first to see if the gaskets have slipped?
Ran several cylinder balance tests. Due to the unstable idle it threw random codes all from the drivers side bank.
Performed compression and noid lit all the injectors with clean results.

At this point I was just going to pull the motor apart, however, I had a feeling it was something stupid, so I reached for my trusty SeaFoam.
Pumped about 4oz into a 1/4 tank and ran the motor at 1500 rpm for about 5-10 minutes until I started smelling the Sea Foam from the exhaust.

The car began to idle normally with 12-15in+(was 8-10in before) vacuum. Still got choppy a few times, but was miles better.

welp, I drove it about 4 hours last night and by the end of the night it was running almost like it should. Thanks Sea Foam!
Take @Noobz347' advice, it is the best so far.

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

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.


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.

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.

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