1999 Chevy 454 7.4 Low oil pressure/rough idle

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Road Trip

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I mean a normal healthy engine might have lower oil pressure, but not where it is tripping the "Check gauges" light at any point during normal operation. Something is definitely wrong. You mentioned that your "chore truck" had a big block.

I'm not sure how similar your engine is to mine, but what RPM does it idle at when hot in Park/Neutral? What about idling in drive?

The factory tach is showing a steady 700 rpm at hot idle. (Which was close enough to the 650 RPM in the manual that I went with it.)
Understand that this is 700 'indicated', not necessarily all that accurate.

Do you have a scan tool to check exactly what rpms it is doing?

Yes I do. I will commit to getting a hot idle RPM reading asap and reporting back with that here
so that you have something to baseline against. (My engine is SES light free & seems pretty happy go lucky.)

Additionally, do you know of any manual that states the proper idle rpms for my engine in those two states?

Here's the Desired Idle speed for the 7.4 per the '99 FSM:

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Speaking of which, you may not be aware that there's a cache of downloadable Factory Service
Manuals covering the range of GMT400 model years. Go give yourself the gift of all the data
that the people who designed your truck pulled together for the dealer technicians to be able to
fix as fast & accurate as possible: (LINK)

After this initial flurry of discovery, we'll get this figured out sooner rather than later.
 

Zatro

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Thousands of years ago, the rude, crude, and socially-unacceptable way to determine gross fuel contamination of the oil was to pull the dipstick, hold the oily end over the flame of a Bic lighter, and see if the oil would readily catch fire.
Hmmm I might try that. It's been sitting for a while tho. Wouldn't the fuel evaporate if it sits for a while?
 

Schurkey

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PCV will tend to remove fuel from oil.

Engine heat will tend to evaporate fuel from oil.

If the injectors are leaking, they'll continually renew the fuel in the oil. It becomes a balancing act. The real-live lab analysis can give a better idea of oil contamination than the Bic lighter.
 

Zatro

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So I'm wondering why the IAC isn't stabilizing/maintaining the idle speed. I'd start by dealing with idle speed and see what happens.
Yeah, I was wondering if the IAC was a reason behind my low idle rpms and rough idle. I tried to clean it with carb cleaner. I unbolted it from the throttle body and started the engine with the wires still connected and the plunger thing pushed all the way out (trying to close off the air because it was letting infinite air in). So, the IAC isn't stuck open, it does move. I was able to determine that.

The only possible issue with it could be that maybe it doesn't fully retract? Because if the plunger doesn't back up far enough then it wouldn't let enough air in at idle for it to reach proper idle speed. Not sure how common that is. Just speculating.
 

Zatro

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Not sure a proper way to test that function of the IAC, but I could do the opposite, where I find a way to completely block the air coming through that passage to the IAC, remove the valve with the wires connected and start the engine and see if it fully retracts, but then the question would remain "How far is fully retracted?". May be some way to determine IAC functionality with a scan tool. Road Trip's picture on post #21 shows "Desired IAC position 60 degrees".
 
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Road Trip

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I mean a normal healthy engine might have lower oil pressure, but not where it is tripping the "Check gauges" light at any point during normal operation. Something is definitely wrong. You mentioned that your "chore truck" had a big block.

That's the '99 C2500 big block 8-lugger that followed me home, and while looking for answers on
sorting out this diamond in the rough, I discovered this forum. Here's a link to my Functional Resto
thread: (Indigo Blue Brutus)

NOTE: What you may find interesting is the fact that it took me 4 passes in order to get the idle
completely sorted. They were nested/overlapping errors, which had me scratching my head,
because I went from a SES light and a pretty steady miss, to the SES light out, but still
intermittently rough, to finally running smooth like new. Just go in a few replies and you'll
start seeing photos of spark plugs & the like.

By the way, the only time I ever see the 'Check Gauges' light is during lthe initial lamp check
prior to the engine lighting off. And the engine has ~228,000 miles on the odo. (Never opened up.)

****

The moral of the story is that you should be able to get your truck to run as nice as
you want it to. Between the General's service manuals and the fact that the talent runs deep
in here, you are in a better place than most other folks driving Brand x vehicles as old as your GMT400.

I'll share the idle RPM from the scan tool as soon as the weather permits.

Cheers --
 
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Schurkey

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Ideally, you'd have scan tool functionality where you can command changes in idle speed. Verifies that the computer can control the idle speed--and that the IAC responds to the computer command, verifying the wire harness and IAC.

I know that this function exists on newer vehicles--I've done it on my '03 Trailblazer. But I don't remember doing that on GMT400s; never had the need. I suspect that function is there but I haven't verified it.
 

Zatro

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@Road Trip Reading through your other forum thread, might have to finish it another day because it's getting late, that is quite a lot of words there. I thought you were onto something with that oil filler cap (mine is also old and loose as you described), but then I got to the end to see that did nothing for you lol.
 

Zatro

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This confuses me. If I'm reading this right it is saying that the oil pressure is supposed to be at 10psi @ 600 RPMs (basically idle) and the check gauges light comes on below 10 PSI? Seems like they should have made the check gauges light come on at a slightly lower pressure if 10psi is basically a normal reading.
Back to this message since we didn't really address it. It's a head scratcher. So, the idle RPM is supposed to be 650 rpms so with a little math (based off 10psi @600rpms) I figured out that at 650 RPMs the oil pressure should be 10.83 psi which I'm just going to round to 11 psi.

Still if the engine is designed to idle at 11 psi under normal conditions and Road Trip is saying that the "Check Gauges" light comes on if the pressure drops below 10 psi, then that seems a bit dumb to me.

I think we all agree that it is not normal for the check gauges light to come on for anyone. But according to that if you are only like 1.5 psi out of spec your "Check Gauges" light is going to come on which isn't very much tolerance.
 

Carlaisle

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Every one of these engines I've had idled right at 650 RPM.

You need to confirm if the oil pressure gauge in the dash agrees or disagrees with a mechanical gauge.

Your fuel pressure is a definite problem. Fixing it may solve the rough running issue. Do you have the OEM injectors in it still? After it has come up to operating temp, if you turn it off for 10 minutes and then attempt to restart it does it need to crank a long time to start?
 
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