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GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 29427
Experience:  GM Grand Master Technician 2007. 14 years experience.
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I have a 02 suburban with a 5.3. I have low idle sometimes

Customer Question

I have a 02 suburban with a 5.3. I have low idle sometimes as low as 200 . It will cause my oil pressure to go to 0. I have changed the MAF ,the PCV , the IAC with new Intake , I have tried everything I can think of. I have sprayed starting fluid every where trying to find a vacume leak. I have even changed the foot pedal sensor . HELP
Submitted: 11 months ago.
Category: Chevy
Expert:  GM Tech (Cam) replied 11 months ago.

Hi there, I am Cam. Sorry to hear about this issue. When you replaced the intake, did you also have a new throttle body with it?

Was there carbon or debris plugging up the ports on the throttle body?

Is there a check engine light set when its idling like this?

Customer: replied 11 months ago.
I replaced the throtal body. Not the intake sorry. Yes there is a check engine light on but it iseems a emissions leak code. From like a bad gas cap
Expert:  GM Tech (Cam) replied 11 months ago.

OK, so code P0442 or P0455, did you check the codes after this issue started happening?

My thought process is that there is a fault with the wiring or connections for the idle air motor and that its not opening or operating. If this is the case, this would be your issue. That motor's job is to let air past the throttle plates to control the idle. We need to verify that wiring and verify operation of the idle air bypass.

Customer: replied 11 months ago.
Also if I park in my driveway facing down hill my oil pressure will drop and turn my oil pressure light on. If I park facing uphill it usually won't drop. And when I stop at a stop light it will usually drop. I have also changed the oil pressure switch. My main concern is the oil . I was thought the two were linked .
Customer: replied 11 months ago.
P0440
Expert:  GM Tech (Cam) replied 11 months ago.

It's all based on idle though. The fact it changes with being parked different ways makes me wonder about loose wiring or connections.

It may also be a bad oil pump or pick up. Have you verified oil pressure with a manual gauge? If not can you please do that and get exact numbers.

Customer: replied 11 months ago.
No I haven't had it checked yet. I didn't figure it was a bad pump cause if I keep it idled at 700 it will stay up. Oil pressure only falls if the rpms drop below 400ish
Customer: replied 11 months ago.
If I keep it idled above 600 the oil pressure won't drop. So I didn't think it was a oil pump. But I can have it checked and let you know
Expert:  GM Tech (Cam) replied 11 months ago.

I agree with that then. So let's focus on the idle. Check out the operation of the idle air motor next.

Customer: replied 11 months ago.
Ok I will check the voltage to it. Cause it is a new one . I'll also check the maf voltage. Cause if I unplug it it runs better kinda
Expert:  GM Tech (Cam) replied 11 months ago.

Do you happen to have a scanner that can view data then?

Customer: replied 11 months ago.
I do . But I am not to sure on what I am looking at when I use the data part of it.
Expert:  GM Tech (Cam) replied 11 months ago.

You would be looking for the desired and actual idle speeds. I do need to know which throttle system to you have, is it the cable or only the electronic throttle? That could change what I suggest and that around the year things changed over.

Expert:  GM Tech (Cam) replied 11 months ago.

Also can you see what the map sensor reading is at idle.

Customer: replied 11 months ago.
I have the electronic idle
Expert:  GM Tech (Cam) replied 11 months ago.

Ok, what is the map reading at idle?

Customer: replied 11 months ago.
8.3-8.6
Expert:  GM Tech (Cam) replied 11 months ago.

What happens to the idle if you unplug that map sensor?

Customer: replied 11 months ago.
It shuddered for a sec but then went back to normal . Kept running
Expert:  GM Tech (Cam) replied 11 months ago.

Did the idle settle to normal then?

Also if you unplug the Mass Air Flow sensor (MAF) sensor, what happens?

And that MAP reading, what units?

Customer: replied 11 months ago.
If I unplug the MAF it runs with a better Idle. .
Customer: replied 11 months ago.
With the key off the map is 28.6 gr with it running the map is 8.6
Expert:  GM Tech (Cam) replied 11 months ago.

What is the g/s on the maF when running?

Customer: replied 11 months ago.
MAF LB/M. .59
Customer: replied 11 months ago.
Now it's. 55
Expert:  GM Tech (Cam) replied 11 months ago.

That fits the idle speed.. Is there any chip or aftermarket devices installed?

Whats your fuel pressure?

Customer: replied 11 months ago.
No after market stuff. My scanner doesn't have fule pressure
Expert:  GM Tech (Cam) replied 11 months ago.

No scanner can read fuel pressure. you need to plug in a pressure gauge at the test port on the passengers side of the fuel rail. Another thing you can also check is the vacuum line on the fuel pressure regulator on the drivers side of the fuel rail on the intake. Is there fuel present in that line?

Customer: replied 11 months ago.
No the fule pressure regulator is good as far as I could tell. No fule in line or fule dripping from regulaitor. I will check pressure a and get back to you
Expert:  GM Tech (Cam) replied 11 months ago.

ok

Customer: replied 11 months ago.
41 psi
Expert:  GM Tech (Cam) replied 11 months ago.

whoa, that’s a little low. Spec is 55-62 psi. If you restrict the return line, does it go back up?

If you unplug that vacuum line from the regulator, what does it go up to?

Customer: replied 11 months ago.
If I unplug vacume line it goes to about 53 psi
Expert:  GM Tech (Cam) replied 11 months ago.

What is the 8th digit of your VIN?

Customer: replied 11 months ago.
Z
Expert:  GM Tech (Cam) replied 11 months ago.

So flex fuel, still that pressure is a bit low. IF you unplug the vacuum line at idle, does the idle return to normal?

Customer: replied 11 months ago.
No change
Expert:  GM Tech (Cam) replied 11 months ago.

OK, spec for flex fuel is 48-54 psi with the engine off, pump running. SO we may be in spec.

So that system is good then at this moment.

So we need to look through this next document.

Customer: replied 11 months ago.
the pressure gauge is jumping around a lot it isn't a solid number
Expert:  GM Tech (Cam) replied 11 months ago.

as long as it remains in that range its good. When running it will drop normally by 3-8 psi. So running if the pressure never goes below 40 then its good.

I would take a moment and check ground wires on the back of the engine block and on the frame under the drivers door.

Customer: replied 11 months ago.
Whathe I don't understand is why when I am on flat ground my oil pressure doesn't drop as fast if it does at all . But if I am on a small hill my op will for sure drop. My gauge in my truck says my idle is 4-500 but my scanner says 6-700 .
Expert:  GM Tech (Cam) replied 11 months ago.

That’s why I am thinking it’s an issue with wiring that moves with the gravity or movement of the truck or possible vacuum lines and such.

Customer: replied 11 months ago.
I don't see any loose wires
Expert:  GM Tech (Cam) replied 11 months ago.

With it running, does wiggling the wiring all around the engine change how it runs?

Have you tried a reset after all this?

Take the battery cables off the battery, hold the ends together for 10 seconds then attach them to the battery and see what happens. This will reset the whole vehicle.
If you have 2 batteries, then remove both and touch only one set.
This reset is more about touching the cables together when off the battery than taking them off. The act of touching them drains the capacitors and forces it to reset.

Customer: replied 11 months ago.
No help.
Expert:  GM Tech (Cam) replied 11 months ago.

OK did you go through the diagnosis I posted in the last document a couple replies ago?

Customer: replied 11 months ago.
No . But my iac and throttle body are new. And I am not sure if my scanner can do some of those test
Expert:  GM Tech (Cam) replied 11 months ago.

Was the throttle body AC Delco or aftermarket?

Customer: replied 11 months ago.
It was a take off of another truck
Expert:  GM Tech (Cam) replied 11 months ago.

Clean the throttle body out then. Use a rag and cleaner to make sure the bore and plates are clean then reset the computer again. I thought the throttle body was new.

Customer: replied 11 months ago.
The throttle body is clean. That was the first thing going I did
Expert:  GM Tech (Cam) replied 11 months ago.

With the throttle not changing it at all, I am left with 2 possible causes. First the MAP sensor on the intake manifold may be failing to read accurately.
They are not that expensive, I would change that.
IF it does not correct it then we may be looking at a bad Powertrain Control Module (PCM). Why the truck being nose down vs nose up makes a difference is strange. I would have thought a loose wire connection, corrosion in connectors or something like that would have been the cause. Its possible there is an issue inside the computer then. A tap test may reveal it. Tapping on the housing of the PCM may cause it to stumble. IF it causes it to change, that’s a sign for sure its bad. IF there is no reaction, that still dow not rule out the PCM.

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