IAC, TPS, Vacuum Leak?

[gavin]

Today started out like any other work-day.
Truck was running fine this morning.
Like every Wednesday, I head on over to a remote site. Truck runs fine on the way there, and the way back to the office.

Of course the way my luck is, when I go leave to head home for lunch, it starts idling horribly.

Now, I'm not sure if I heard any odd noises or not at initial start-up, but now I'm hearing what almost sounds like a hiss. Now, this hiss does change with a change in RPMs.

Idle is high. Around 1200 I would wager. Also, when shifting from D to N, it jumps up to 2000 RPM for about 5 seconds or so before it slowly drops back down to 1200.

When stopped in D, it's a rough idle.

Now, when I disconnect the MAF, idle is good. A bit high, but good.
While at home on lunch, I cleaned the MAF and unplugged the PCM for a bit. No change.
I have an aftermarket Granatelli MAF on, so I swapped in my OEM. No change.

Also, shifting feels.... strange. It seems to shift at odd times compared to the "usual"

I'm guessing IAC, TPS, or possibly vacuum leak? Although vacuum leak wouldn't be likely if the hiss noise does not change with a change in RPMs, correct?

This would be on my '97 5.0.

 

[SkyJumper]

In my past experiences with IAC, it usually effects idle or start up capabilities, especially after engine is hot and you get in to restart after shut down. I would be taking a bottle of carb cleaner and spraying around all vac lines and intake manifold seals and see if where I spray increases idle or effects idle. I have always had good luck with a spray can of carb cleaner in locating a vac leak. Your aftermarket MAF sensor I would not tend to think it is the culprit. But a bad vac leak will effect the shift points of the trans as the increase in RPM'S is telling the ECU that you are in need of kick down in shifting. Also spray around the brake booster vacuum line also to the manifold.

 

[gavin]

In my past experiences with IAC, it usually effects idle or start up capabilities, especially after engine is hot and you get in to restart after shut down. I would be taking a bottle of carb cleaner and spraying around all vac lines and intake manifold seals and see if where I spray increases idle or effects idle. I have always had good luck with a spray can of carb cleaner in locating a vac leak. Your aftermarket MAF sensor I would not tend to think it is the culprit. But a bad vac leak will effect the shift points of the trans as the increase in RPM'S is telling the ECU that you are in need of kick down in shifting. Also spray around the brake booster vacuum line also to the manifold.

yeah, didn't have any kind of carb cleaner or starter fluid so didn't try that at lunch.
Since I don't usually get home 'til 5, would have liked an idea as to sensors (if needed) so I could pick them up on my way home.

If it was a vacuum leak, would the idle smooth when MAF is disconnected, and get back to rough idle when plugged back in?

 

[SkyJumper]

yeah, didn't have any kind of carb cleaner or starter fluid so didn't try that at lunch.
Since I don't usually get home 'til 5, would have liked an idea as to sensors (if needed) so I could pick them up on my way home.

If it was a vacuum leak, would the idle smooth when MAF is disconnected, and get back to rough idle when plugged back in?

Usually a MAF unplugged will cause normal RPM cycle to change, mostly to a lower side. But, just because a MAF is being unplugged and RPM drop happens instantly does not correspond to a faulty sensor. It just means that you are breaking the communication path to the ECU. The ECU is built to compensate for fuel and air deliverance, and if it detects that there is a issue, the ECU will command compensation elsewhere to try and achieve optimal or normal performance. So, to answer your question, I would not jump right away at a MAF sensor.

Now I on the other hand seem to go and buy all new sensors when I feel some or just one are working improperly. But I only do that to give myself a record of replacement for future information. But that can be expensive. I would first get a can of carb cleaner, spray the engine down area by area and determine if there is a vacuum leak, then start taking a multi-meter to the sensors.

Are you receiving any CEL codes?

 

[gavin]

ok, after thinking about this and re-reading SkyJumper's post, brake booster is certainly a possibility.
It does seem that the "hiss" I'm hearing is coming from the driver's side area.
And this has a constant amount of vacuum to it, regardless of engine RPM correct? Which would explain why the hiss doesn't change when RPMs do?

 

[gavin]

Usually a MAF unplugged will cause normal RPM cycle to change, mostly to a lower side. But, just because a MAF is being unplugged and RPM drop happens instantly does not correspond to a faulty sensor. It just means that you are breaking the communication path to the ECU. The ECU is built to compensate for fuel and air deliverance, and if it detects that there is a issue, the ECU will command compensation elsewhere to try and achieve optimal or normal performance. So, to answer your question, I would not jump right away at a MAF sensor.

Now I on the other hand seem to go and buy all new sensors when I feel some or just one are working improperly. But I only do that to give myself a record of replacement for future information. But that can be expensive. I would first get a can of carb cleaner, spray the engine down area by area and determine if there is a vacuum leak, then start taking a multi-meter to the sensors.

Are you receiving any CEL codes?

I know the MAF is not the culprit
but I know it means something when the idle smooths out when the MAF is disconnected :

no CEL. Although it was a short trip home; 10 minutes tops, about 4 miles. May not have been long enough to trigger CEL. Plus, I let the computer reset while at home for lunch.

I s'pose I can always buy the sensors and just return 'em if I don't need/use 'em.
I'll try and take a look at the brake booster hose before I leave since I did bring my flashlight with me.

 

[SkyJumper]

ok, after thinking about this and re-reading SkyJumper's post, brake booster is certainly a possibility.
It does seem that the "hiss" I'm hearing is coming from the driver's side area.
And this has a constant amount of vacuum to it, regardless of engine RPM correct? Which would explain why the hiss doesn't change when RPMs do?

The vacuum leak will aways remain constant, regardless of RPM range. If the hiss is loud, the brake booster is where the largest demand with vacuum is. The hose going from the brake booster to manifold is I think 1/2" rubber line. It can crack, or leak at the weak ass press clamps the manufacture uses. I always replace them with hose clamps, but be sure if you replace them that you do not over tighten them as the fitting on the brake booster is only hard plastic, if you over tighten, then you are collapsing the fitting to restrict vacuum pressure.

 

[SkyJumper]

If you have time before you leave for home and am able, let your truck running park for about 30 minutes, the RPM increase will not harm the engine while running like that. But what it will do is cause a CEL if there is one. I faulty sensor that is reading out of ECU tolerance will come on. But it usually takes 10-20 miles of driving after reset or constant running for about 30 minutes.

 

[gavin]

thanks SkyJumper.
A leak at the brake booster would make sense.
As when I started the truck, I swear I heard a pop or something. Obviously I may not have, or it may have been something else, but that could make sense if the hose popped off or broke.

I try not to hang out at work too long after I get off : gotta get home to let my dog out.
Definitely sounds like a possible culprit though...

So neither the IAC or TPS would cause my symptoms?

 

[SkyJumper]

thanks SkyJumper.
A leak at the brake booster would make sense.
As when I started the truck, I swear I heard a pop or something. Obviously I may not have, or it may have been something else, but that could make sense if the hose popped off or broke.

I try not to hang out at work too long after I get off : gotta get home to let my dog out.
Definitely sounds like a possible culprit though...

So neither the IAC or TPS would cause my symptoms?

None of the sensors will cause a hissing noise, only a VAC leak. And if you can hear it in the vehicle while driving, then I tend to beleive you have a VAC issue and not a sensor issue. And a vac leak can happen at a moment notice, but a sensor going bad usually happens over a period of time and the syptoms would of told you so before this.

 

[Turdle]

Also check the PCV grommet at the rear of lower intake. Check the hoses running to the upper plennum. You might have had an intake backfire blow a line loose. The brake booster was a good call also. Try moving the grey one way valve conneting the hose to the boster while the engine idles and listen for a vacuum hiss noise.

 

[gavin]

I'll try and check the PCV valve. That was definitely a PITA to replace, but I may be able to manage to get my arm back there again.

I should be able to take a gander at part of this before leaving work.

Thanks fellas!

 

[gavin]

ok, so here's what I noticed on my way home.

started up the truck, and it idled very rough and sounded like it was about to die.
went ahead and disconnected the MAF, idled smoothed out at probably just over 1000RPM.
Doesn't appear there is a leak at the brake booster; jiggled the hose at the gray valve and nothing.

Possibly sounding like the hissing is coming from behind the engine, which would/could be the PCV valve.

Don't have any carb cleaner or starting fluid, but sprayed some intake cleaner.
Sprayed around the valve on the booster, and nothing.
Sprayed some behind the engine, and nothing.

No CEL yet, but I will scan it anyway just to see if there are any codes pending.

 

[gavin]

P1506 - Idle Air Control Overspeed

would disconnecting the MAF cause this code?

and FWIW, the "hiss" I hear, is possibly a pulley. Certainly will not rule that out. It appears to come from behind or dri side of the engine, but I am not 100% positive.
Although then the sound would change with RPMs... hmm...

 

[Turdle]

Could it be coming from the purge solenoid under the battery tray? There are some vacuum lines going there from the upper intake.

p1506 means the idle did not drop when the IAC was signaled to be closed.

 

[gavin]

Could it be coming from the purge solenoid under the battery tray? There are some vacuum lines going there from the upper intake.

hmm.. did not know that. Stupid vacuum routing diagrams appear to be non-existant in the FSM, and the Chilton's is no better than the sticker under the hood.
Will try and check that out while the engine is cooling off.
already slightly burned myself trying to reach for the PCV. whoops.

 

[gavin]

ok, managed to get to the PCV valve. It's in good; hose is good as far as I can tell.
Hooked MAF back up.
Started up; prior to firing, I heard a couple "pops"

the hiss is definitely coming from the driver's side, and it sounds like it may be coming from closer to the front of the engine, possibly around the evap service port area?

 

[gavin]

oh, and VERY RICH exhaust. Very strong smell of gas.

 

[SkyJumper]

Have you taken the IAC off and look inside it to see if it is all gunked up with carbon? Try spraying it out and then reinstall it.

 

[gavin]

Have you taken the IAC off and look inside it to see if it is all gunked up with carbon? Try spraying it out and then reinstall it.

yup.
I actually did that prior to my last post.
no change...

it didn't look TOO bad.