Best Way To Find a Vacuum Leak
 

Title says it all.....What is the best way to find a vacuum leak? One that is big enough to keep the truck from running below 1500 rpm.

Well, if it's that large of a vacum leak you should just about be able to hear it. Failing that, one of the older tricks is to spray the vacum lines and other vacum locations - like the intake plenum, brkae booster, vacum tree on top of the intake - with WD40, carb cleaner or something like that. As soon as you hear the engine change RPMs then you should have found the vacum leak. Some of the braver souls on this forum have used a propane flow from one of those disposable cylinders to search for a vacum leak. The same idea applies. As soon as the propane gets sucked into the combustion chambers the idle will change and there is your leak, or at least you'll be close.

Is there a specific reason that you think that the idle problem is strictly vacum related? Have you tried to pull the codes from the ECU?

Post back with maybe some more details about your problem and see if there are any more thoughts about what could be going on.

Cheers!

Mike

You can also pinch off vacuum lines at their source. If that finds it, trace the line to the componant.

Do not use highly flammable gasses or liquids, you may compound you problems. Anything can burn, but as MRL123 says, WD40 works and has a high flash point.

Check your EGR also.

Thanks for the replies.


I have been fighting this for 3 or 4 weeks. I pulled the codes, and got a code 31. I replaced the egr valve and the egr valve position sensor hoping that I was killing 2 birds with 1 stone...addressing the cel and my idling problem at the same time. As it turns out, the check engine light went away, but the idling problem did not. :(

The EEC now gives me the "system pass" flashes (11 11)...so no help there. Well, actually I did get a code 14 in the stored memory part of the KOEO test. Could that mean anything? I double and triple checked all the vacuum lines and have yet to find anything disconnected, cracked, broken, mis-routed or anything else that would "throw a red flag." I can't hear anything either.

As far as more details, the engine is new, as are all the actuators and sensors and just about everything else under the hood for that matter. The only things that are not new are the EEC (if I don't get to the bottom of this soon, I may replace this one), air pump, some of the plumbing for the air pump, wiring harness, vacuum harness and I think thats about it. All the sensors are checking out according to specifications given by FFI.com.

When I first started the engine up for initial break in, it ran VERY smooth. I have started the engine up 3 or 4 times since then and it seems like each time I start up it runs a little bit more "rough." It shakes a little bit, the RPM seems to surge a little bit and it is also running cooler...the last one has anything to do with my problem, but just an observation.

The reason that I am thinking that a vacuum leak may be the culprit (or maybe the EEC, but I will look for a vacuum leak first) is simply because I cant think of anything else. There are no codes and everything is checking out fine via my DVOM, so I am considering the possibility that I left some vacuum line unplugged and that is the source of my problem...hence this thread.

Any input as to what I ought to do to get to the bottom of this would be appreciated.

Thanks!

Check manifold gasket?

Running a little cooler? What temp? could be related.

Manifold gasket is good.

The first 2 times I started up, the mechanical temp. gauge read at the highest point of the "normal" section, pointing towards 2 o' clock. Now it is reading in the middle of the normal section...it is pointing at 12 o' clock. I heard that was normal for new engines.

I also have access to a 1990 F250, same motor, that I have been using to help me diagnose. I could not make it act like my 1988 1/2 ton 302 no matter how much of a vacuum leak I gave it. So, that leads me to believe that I am not dealing with a vacuum leak. As a matter of fact, I cant make it act like the 1988 no matter what I do. The vacuum leaks that I gave the 1990 resulted in a "stumble" then an increase of RPM.

Two common leakers are the blend door line, and the "coffee can".
The blend door line supplies vac to your cab temp controller, the door can be set at various positions to allow all air to flow thru the heater core or vary that amount to get less hot air. The line is white in color and can be seen in the area of the blower box, in the engine bay, passenger side. The line seems to rot out and develop leaks.
The coffee can is a metal cannister on the pass side fender wheel well that is supposed to act as a vacuum reservoir; if you are at low idle your engine vacuum suffers, and one of the various vacuum controlled devices may need a little vacuum to control something. This cannister provides that "extra moment" of vacuum to keep the system stable.
The coffee can is coated with one coat of paint, and it isn't long before the can develops serious leaks.
There is also a vacuum reservoir mounted under the foil insulation around the heater blower box. Ford hid that one very well, took me 10 years to figure out it was there.

would think of vacuum leak first. But How old is your o2 sensor? I had an engine stumbleing and boggin problem with mine. Ckecked all my vacuum lines and they were good. Then I heard about the o2 sensor could be going bad or dirty but sometimes doesn't throw a code. Changed that and my problems were cured.

Just got done using propane and starting fluid to search for a vacuum leak and the only place that caused an increase in rpm was the air intake hoses themselves LOL.

My 1988 doesent bog and stumble at all. It just dies INSTANTLY when letting off of the throttle...hot, cold, whatever. I will look into the O2 sensor though. One reason that I did not think to look there is because the O2 does not do anything untill you are in closed loop...my problem is present in opened or closed loop.

Keep the ideas coming

Thanks again

check your tps and idle air control valve, could be part of the problem or possibly the coolant temp.sensor

I replaced the O2 and I took the ATC and the ECT out of the 1990 and plugged those sensors into the plugs on the 1988 without removing the sensors. So, the ATC and the ECT sensors are reading the air temperature near the engine...close enough for testing purposes. The other ATC and ETC sensors are still in their place, but they are unplugged. Once I did this, the engine will now idle...but the idle surges from 600 to 1100 rpm. If I put a slow stream of propane into the air intake hoses, it smoothens out alot. I think I am on to the right track. My next move: Remove the ATC and ETC from the 1988 and bring them back to NAPA for a refund and purchase new Motorcraft ones from Ford. I am learning the hard way that OEM is the way to go.

I will keep y'all posted

Thanks

Sounds like your on the right track.

I also found out the hard way about using OEM parts.

My EGR sensor went bad. Got one from Auto Zone. That one failed 2 weeks later. They would not refund my money on the failed part. But they did offer to exchange it for a new one. I said "no thanks" and paid the extra for the OEM sensor. That fixed it.

I hope you can return the sensors. Many auto parts stores do no take returns on electrical parts.

Keep us informed....

Leaks/vacuum hoses/excessively high idle ---???
 

Hi,
I've been following this thread with great interest. I recently had to remove the upper manifold to change out my valve cover gaskets. A pain in the neck that I'm glad is over, or so I thought.
Once I got everything back together and cranked up the truck. Idle was at the "high" cold engine rpm's. But it never kicks down now. Never had this problem before...
Could this be a vacuum issue? I thought I was extremely careful about labeling and rehooking all back up the way it was but accidents do happen.

Any ideas are greatly appreciated.


Thanks,
Cjax

Cjax, sounds like a vacuum leak to me. Thats exactly what the 1990 truck (truck that runs well) did when I gave it a vacuum leak while trying to make it act like the 1988 truck (the one that I am struggling with).

I replaced the air temp sensor and the ECT...and nothing changed :( I thought I was on the right track, but I guess not. So, back to the drawing board.

The only way to keep this motor running without touching the throttle is to feed a stream of propane into the air cleaner. But, that only works for maybe a minute. Once the minute is over, the rpm surges from 600 to 1100 give or take. The funny thing is that this problem was not present before the motor was removed. The motor had TONS of blow-by and a couple of cylinders had no compression. That was the reason for the overhaul. I thought to myself..."Well, as long as the motor is out, this is the perfect time to replace everything else!" Seems logical, right? So, in come all the new sensors, actuators, distributor etc. etc. Once everything was in place and ready to go, I did my break in. Everything seemed alright during the 20 minutes that the motor was running @ 2000 rpm. Once I was ready to shut the motor down and proceed with the oil and oil filter change I let off the gas and reached for the ignition, but before my hand got to the keys the motor was already shut off. That was when I first noticed the problem, and ever since then I have been fighting this.

The million dollar question: What could have happened while replacing EVERYTHING that would result in a problem like this?

I wish that the 1990 F250 had the same EEC so that I could try it out on the 1988, but it doesent. More and more I am wanting to just buy a new (well, not new but "remanufactured") EEC and hope that it fixes my problem, but everything that I read tells me how indestructable these things are and that 99 times out of 100 replacing the EEC does not fix the problem.

So, what do I do?