Need help! Possible vacuum leak or bent valve.

[88CougarMan]

Hey guys,

I think I have a vacuum leak somewhere or possibly a bent valve. If you read down, I sent a private message to jrichker and he told me to start a thread so he can post some diagrams and get other people's insite. Read below and you'll see what all I have done. There is a chance that I didn't do the cylinder balance test correctly so a bent valve isn't completely ruled out. Can anyone help me pinpoint some common areas where a large vacuum leak would occur? I can't visibly see any major tubing that may be cracked or broken.

Please help!

Thanks in advance!



I recently purchased a '91 5.0 and it wasn't running right. It was running lean (popping through intake) stumbling, and throwing codes. After pulling the codes and getting a whole slew of them I fixed them all over the past few weeks. Most of them were sensors and vacuum lines hooked up wrong. After finally getting a system pass code 11 and no more check engine light, the car still backfires through the intake at idle and under load. All this after checking the timing, replacing the MAP, ACT, ECT, EGR ,O2 sensors and replacing the fuel pump. This stumped us, so we did the cylinder balance test and the test light came back with a 9, which, as you know, is all cylinders OK. Finally we put a vacuum gauge on the intake manifold and at 1000rpms we are only reading 5-10 inches of vacuum pressure. So, my question to you, in your expert opinion, after hearing all of this what do you suppose the problem is? Vacuum leak or bent valve/bad spring? Would a vacuum leak cause the popping through the intake? I know a valve hanging up would.

 

[88CougarMan]

Now that I think about it, when the car is running and I push the brake pedal, it almost goes to the floor and I get sporatic surges of pressure pushing back on the pedal. That leads me to lean towards a vacuum leak. Thoughts?

 

[ID89GT]

Something you could try is removing vacuum lines that go off the intake and plug them off one by one, that way you can see if it changes when something is disconnected.. Also the brake thing makes me wonder if the brake booster is a possibility, but what do i know

 

[88CougarMan]

So, I was going over all the vacuum hoses and ran into the PCV... it was completely unplugged. The screen down in there is caked in oil and large chunks of dirt. So, I ordered a whole set up, but my question is: Would an unplugged PCV be the cause of my troubles?

 

[Strype]

Umm... Blow by. I could see that. Isn't the PCV hooked up to a vacuum line?

 

[jrichker]

Sucking in air other than what it is supposed to do will cause problems.

 

[88CougarMan]

I cleaned up the PCV, filter and grommet, stuck it back in and started it up to see if that was causing the problems. It didn't help it. The symtoms are, on a cold start:

it fires up ok, immediately wants to die but maintains running with a really low rpm, you'll hear a pop or two through the intake with no throttle applied. If you try and give it throttle the pedal will go down with no increase in rpm then a pop and the rpms shoot up to 1500 or so. It is damn near impossible to maintain the rpms at 1000. When the engine warms up the idle and the throttle is smoother but the popping still continues more sporatically.

???

 

[jrichker]

Check the vacuum gauge readings again. You may still have a vacuum leak somewhere.

 

[FastDriver]

A compression test can help to identify a bent valve, and would rule out a vacuum leak assuming you only have one problem.

Chris

 

[jrichker]

A bent valve, broken valve spring or valve adjusted too tight will give a sweeping indicator reading on a vacuum gauge. The reading will fluctuate 5"-7" of vacuum every time the cylinder comes up on compression.

A cylinder blanace test will also help to spot a bent valve, broken valve spring or valve adjusted too tight.

Cylinder balance test:
Warm the car's engine up to normal operating temperature. Use a
jumper wire or paper clip to put the computer into test mode. Start
the engine and let it go through the normal diagnostic tests, then
quickly press the throttle to the floor. The engine RPM should exceed
2500 RPM's for a brief second. The engine RPM's will increase to about
1450-1600 RPM and hold steady. The engine will shut off power to each
injector, one at a time. When it has sequenced through all 8 injectors,
it will flash 9 for everything OK, or the number of the failing cylinder
such as 2 for cylinder #2. Quickly pressing the throttle again up to
2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is
weak and isn’t putting out power like it should. See the Chilton’s Shop
manual for the complete test procedure

Dump the codes and see what the computer says is wrong…Codes may be present in the computer
even if the Check Engine light isn’t on.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check
engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing
test lamp or Check Engine Light and count the flashes.
or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the
test jumper in place. Be sure the A/C is off and the transmission is in neutral. You'll get an 11, then a
4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle,
it will dump the engine running codes.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light,
or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or
Check Engine Light and count the flashes.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical

If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional
check engine light on the 86-88's except possibly the Cali Mass Air cars.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the
self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the
self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the
12 volt power to the under the hood light. Do not jumper it to the computer test connector.
If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings
are OK, and that the wiring to put the computer into diagnostic mode is good.
No code 11 and you have some wiring problems.


Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder
balance test. Low compression on one of these cylinders rules out the
injectors as being the most likely cause of the problem. Look at cylinders
that fail the cylinder balance test but have good compression. These
cylinders either have a bad injector, bad spark plug or spark plug wire.
Move the wire and then the spark plug to another cylinder and run the
cylinder balance test again. If it follows the moved wire or spark plug,
you have found the problem. If the same cylinder fails the test again,
the injector is bad. If different cylinders fail the cylinder balance test,
you have ignition problems or wiring problems in the 10 pin black &
white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark
plug hole. The other type leaks too much to get an accurate reading.
Your local auto parts store may have a compression tester to rent.
If you do mechanic work on your own car on a regular basis, it would
be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the
throttle wide open, crank the engine until it the gage reading stops
increasing. On a cold engine, it will be hard to tell what's good &
what's not. Some of the recent posts have numbers ranging from
140-170 psi. If the compression is low, squirt some oil in the cylinder
and do it again – if it comes up, the rings are worn. There should be
no more than 10% difference between cylinders. Use a blow down
leak test (puts compressed air inside cylinders) on cylinders that
have more than 10% difference.

See the link to my site for details on how to build your own blow
down type compression tester.

 

[88CougarMan]

So I redid the cylinder balance test twice and came up with some confusing data.

1st Test:

9, 9, 6...1...6...1

2nd test:

8...1...8...1, 6...6, 8...6...8...6

What the hell does that mean? Saturday I am going to check the plugs, which were the first thing replaced before I started testing and trouble shooting the engine, and see what they tell me. Maybe the injectors aren't firing correctly... call me crazy, but I wouldn't think a malfunctioning injector would cause the pop through the intake? I would think it would just cause the engine to run rough and throw a code to the O2 sensor.

After that I'm going to do a compression test on all the cylinders.

 

[jrichker]

The CBT will test all the cylinders in sequence. After it drops the RPMs back to idle it flashes the results. The first single digit it flashes is the results of the test. It will continue to flash the Engine Running codes it found after the report of the CBT.

The 9 was a pass in the CBT. The other numbers after the 9 could have been code 61, ECT out of range.

Since I was not present for the sencod pass of the CBT, I am not sure what you got for results.


Code 61 - Engine Coolant Temperature (ECT) sensor is or was too low. Failed sensor or bad wiring for the ECT.

]Note that that if the outside air temp is below 50 degrees F that the test for the
ECT can be in error.[/b]

Check the resistance of the black/white wire to battery ground. If it is less than 1 ohm, it is good. If it is more than 1 ohm, the black/white wire has bad connections or a broken wire. Always take resistance measurements with the circuit powered off. Make sure that you do not touch the metal probe tips when you you’re your measurements. If you do, your readings will be off. Check the resistance of the Lt green/yellow wire at the sensor connector and Pin 7 on the computer. It should be less than 1 ohm. If it is more than 1 ohm, the Lt green/yellow wire has bad connections or a broken wire.

If those tests pass, remove the passenger side kick panel. Disconnect the computer connector and disconnect the sensor from the wiring connector. Measure the resistance between the Lt green/yellow wire at the sensor connector and ground. You should see 1 meg ohm or an infinite open circuit. Less than 1 meg ohm means you have some bad wiring, or the Lt green/yellow wire is shorted to ground.

The ECT sensor has absolutely nothing to do with the temperature gauge. They are
different animals. The ECT sensor is normally located it the RH front of the engine in
the water feed tubes for the heater.

The ACT & ECT have the same thermistor, so the table values are the same

ACT & ECT test data:

Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate
readings.

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Voltages may be measured across the ECT/ACT by probing the connector from
the rear. A pair of safety pins may be helpful in doing this. Use care in doing it
so that you don't damage the wiring or connector.

50 degrees F = 3.52 v
68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v

Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.

50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 k ohms

Diagram 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 Mustang FAQ - Engine Information

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Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
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Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg