Vacuum leak in 2002 Toyota Corolla



The car is a 2002 Corolla with auto air, trans and 85,000 miles. About 2 months ago the check engine light came on. Pep Boys checked it and said it had “intermittent and random misfires”. I changed the plugs and one ignition coil I put the new coil on all four plugs. No cure. A recheck and the technition said “intermitent and random misfires and running lean” at my local auto parts house.

I took the car to Pep Boys again and the master mechanic ran an engine diagnois for $75.00. His conclusion was the car has an vacuum lean and he couldn’t locate it though he tried for an hour! No charge by Pep Boys since they couldn’t fix the problem. I drove around to the back of the shop and gave the mechanic $30 since that was all I had on me.

Help! I’m a poor preacher…No Tee Vee show…and I use this car as a hospice pastor.


Yeah right.
That’s why you drove around back and gave the guy $30.


I don’t know haw many real “Master Mechanics” work at Pep Boy’s, but one of the more difficult vacuum leaks to find is an intake manifold gasket leak. It is usually found by spraying some carburetor cleaner all around the gasket looking for a sudden change in idle. You could also spray around the throttle body and maf sensor gaskets if the intake manifold gasket appears to be ok.

You could be having a fuel injector problem, but these are usually pretty reliable.


A vacuum leak will generally cause a rough idle or a lean surge at speed, depending on where and how severe the vacuum leak is.
A vacuum leak is easily determined with the use of a vacuum gauge; cheap and easy.

It sounds like you’re not mechanically inclined and this yo-yoing back and forth between an auto parts house (AutoZone?) and Pep Boys is creating problems.

You’ve purchased one new coil, but how certain are you that one of the remaining ones is not bad?

Not much I can add unless you have someone read the codes and post them exactly as given here; with no interpretations by the hired help there.

ok4450 logged out again


Roadrunner, I don’t understand your reply. If you’re insinuating I did not pay the mechanic $30 you’re wrong. I did. I’m a pastor. I don’t lie and I don’t expect some guy to work an hour on my car and not pay him. Pep Boys doesn’t pay their mechanics a thing if they can’t fix a problem. That just doesn’t seem right to me. That man worked hard to get certified in all the areas he is certified in and having spent 3 years in seminary after college I can appreciate what he did. If I’m off base with this reply it’s because I don’t understand what you were implying. My apology in advance if I have offened you in some way. Blessings…HIS !


I’m sure that mechanic appreciated your kindness Jon and He did to. It amazes me He sees all these things, good and bad. I wouldn’t be surprised if you get an unexpected return for that.

As suggested OK4450, the use of a vacuum gauge would be a big help in finding the leak. If the manifold gaskets aren’t leaking then maybe something like the brake booster or vacuum modulator is. By plugging off suspected areas the leak should be fairly easy to find. I suppose the leak could also be in the exhaust area before the O2 sensor.


While it may not be widely known, many mechanics wind up doing a lot of things for free no matter if it’s PepBoys, independent shop, or a new car dealer. The latter is the worst offender; especially if warranty is involved.


This is my second try at replying Cougar so if you’ve read this already, I beg your pardon. I printed your suggestions like the others. Now I’m going to buy a vacuum gage as Keith, OK4450 and Anonymous suggested and see what I can do. And thanks for your kind comments. I find that living by the Golden Rule is best.


It’s a shame that more mechanics don’t use a vacuum gauge. They’re cheap, easy to use, and can tell you so much once you learn to read it. (A new gauge will come with a chart).

A pair of needlenose pliers can be used to pinch off hoses while searching for a leak, but I use some old surgical forceps. No sharp edges that might inadvertently crack or cut a hose.


If the egr valve is allowing exhaust gas into the engine at idle, or other “wrong” times, it will LEAN the fuel/air mixture. The Idle Air Control valve, by order of the ecm (engine computer), or by malfunctioning, can allow excess bypass air into the engine. ++++ The egr valve, and the IAC valve, can usually be disabled by disconnecting their wiring, or fuse, or vacuum hose. Doing so, will, of course, set trouble codes in the ecm (check engine light).


More info.

I went back to my auto parts house and they checked the codes for me. I got two.

P0171 with the explanation of “System too lean” and
P0300 withe the explanation of "Random multiple misfires.

If anybody has anything more to offer in the way of help before I buy a vacuum gage I would appreciate it. Jon


An engine which is running too lean WILL misfire.




[b]Here’s something you might want to try. You have the codes and their definitions.

Most library’s have free access to the ALLDATA database. ALLDATA is an auto repair manual on the internet.

If you go to this site, and find those codes for your vehicle, it will provide flow chart on what to check that could be causing those codes. You can even print them out.

Now I’m not saying that you should check this stuff out, but with that information in hand, you have a better chance of finding the problem.



To clarify, no one is in disagreement. We’re all saying that too much air is going into the engine for a given amount of fuel. We have listed some of the different ways in which this extra air may be getting in: by vacuum leaks in different areas and/or excess air through other means. These leaks will show up as lower than expected vacuum. Not named before, the joint (gasket) between parts like the air intake manifold and cylinder head could be leaking, and so on. +++ Something to try, is to get the sealant known as RTV sealant (the person at the auto parts store will know), and to smear it on every joint on the top of the engine. If one of those is leaking, the RTV will seal it.


Thanks a lot Tester. That’s really cool information! I didn’t know about AllDATA. Hopefully I can get down to the library this weekend.


I think I used something like what you’re talking about when I was a young pup only we called it liquid gasket. It was dark brown sticky stuff and it did work! After I check ALLDATA and if I think I can do the work I’ll do as you suggest and get some RTV. Thanks for the reminder.


The Saga of the Sick Corolla continues!

I went to my local library was told they didn’t subscribe to AllDATA. At that point I tried spraying some carburaetor cleaner around the intake manifold, the throttle body, maf sensor, and anywhere else I thought there might be a vacuum leak but the engine idle didn’t change. So I abandoned any effort at trying to fix the problem my self and went to the local Toyota dealer.

First try, the tech cleaned the throttle bore. They confirmed the codes I was getting P0171 and P0300 or random multiple misfires and system running lean. Cost was $77 The engine check light came back on 4 days later. Back to the shop.

Second try, the tech replaced the mass air flow sensor. Codes were the same. Cost was $281. The check engine light came back on again today 8-14-07.

The service manager seems like a nice guy and told me to bring it back on Thursday 8-16-07 and they will work on it for no cost.

If anyone has any new ideas I would like to hear from you before Thursday 8-16-07. Thanks in advance, Jon.


As Tester said theres a flow chart for every trouble code.

From the factory shop manual for my 02 Sonata:

Trouble code p0171: Fuel system too lean

Possible causes:

1 "Faulty ignition system (ignition coil/spark plug/ignition cable)

  1. Faulty fuel delivery system (Fuel tank/Pressure regulator/Canister purge valve.)

  2. Clogged fuel injectors.

  3. Faulty fuel injectors.

  4. Vacuum leak in intake system

6.Leak in exhaust system.

  1. Faulty MAF sensor."

If you’re lucky number 3 is the problem & you might solve this with a bottle of injector cleaner added to the gas.


Forgot to add this & since the dealer replaced the MAF sensor…

“If any codes relating to injectors 02 sensors, ECTS, or MAF sensor are stored, do all repairs associated with those codes before proceeding with this trouble area.”

Yours did’nt have a MAF code, but the dealer replaced it anyway.

Like i said this is for my 02 Sonata, but yours should be very similiar.

Considering whats been checked or replaced 2. 3. & 4. look like the most likely culprits.

If fuel pressure is low (number 2)that will cause the lean condition & random missfires.