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Old 02-21-2005, 07:03 AM
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Open Loop troubles (long post) Revisited

Alright, previously i posted a question about closed loop operations and sensors. Open Loop troubles (long post)
i now have real-time sensor data i can provide.

this set of data was saturday morning the weather channel said that the temp. outside was 20 degrees F and the trucks mirror display said 21 degrees F.

The first thing that got my attention is the seemingly low manifold air pressure (well be low a resonable 15 psi).

With this data does anyone have any suggestions. Thanks in advance (pun intended)

I realized after posting that the columns may not line up for someone else so i've added a picture of the data
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Old 02-21-2005, 08:08 AM
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That thread link is not working.

Please clarify your question. By "Open Loop Troubles", do you mean that it will not enter into Closed Loop? If so, remember that it will not enter closed loop until a set of parameters are met as defined by the computer. Based on the attached scan results, it appears you have not been met that criteria. Generally, a min. coolant and/or manifold air temperature must be met. Then a min. RPM, generally about 1500 RPM.

Based on your data, the engine was only running 136 seconds, the coolant temp only reached 75*F, MAT 32*F, and you were idling around 825-850 RPM.

Now, I generally understand manifold air pressure in terms of kPa. Converting your 7.105 psi to kPAa (multiply psi by 6.8947), I get about 49 kPa. A normal engine will idle in a range of 35 -50 kPa, depending on the age of the engine and/or size of the cam installed. Bigger cam = higher kPa (i.e. lower vacuum). My stock 94' Suburban 5.7L engine idles about 40-45 kPa.

I see that short term fuel trim is adding fuel, which is to be expected for cold starts, especially at 20*F ambient air temp Your TPS seems a little low at .39 for an idle. It should be closer to .50-.60 depending on the vehicle. Speaking of which, what engine and fuel injection system (i.e. TBI, etc.)? I see some readings for MAF (mass air flow), so I will assume you have a Vortec 350? in this Suburban?

Let the engine fully warm up and then take some readings. Ed www.edgesz28.com

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Old 02-21-2005, 08:36 AM
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Ok, the information was taken upon initial startup on Saturday morning. The truck, a 2000 Chevy Suburban with a 5.3L Vortec, MPFI, runs like absolute crap (notice the RPM's jumping around)and will even die until the system begins to rely on the O2 Sensors. Then idle improves, but is still rough.

When i previously posted this problem, many said additional information is needed about the sensors so that is what i posted, however i have some other data later in the run when things are getting up to temperature and running better.
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Old 02-21-2005, 09:58 AM
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Quote:
Originally Posted by tnoftsger
The truck, a 2000 Chevy Suburban with a 5.3L Vortec, MPFI, runs like absolute crap (notice the RPM's jumping around) and will even die until the system begins to rely on the O2 Sensors. Then idle improves, but is still rough.
Okay, now I notice something else. Look at your advance readings. Even though your idle RPMs are only flucuating 50 +/- from 600 RPM, your advance is jumping from 24* down to 14*-17*. What's causing this? Perhaps you have an ignition problem (coil, plug, wire). Or perhaps something else like a fuel injector(s) or IAC (idle air controller) is bad causing the fluctation in the engine's idle.

Considering it is a 2000 vehicle, I doubt very seriously that it has anything to do with the stock code or a bad PCM. It is either getting bad information in (sensor) and outputting bad information the device; or some device(s) itself is faulty (coil, injector, IAC, etc.)

The computer has tables that establish the timing advance, with various variables that either add or subtract advance, depending on certain criteria. For instance, cold start will add fuel and advance and pull it out gradually as the engine warms up. I would guess that in the idle range, it is set for 24* BTC (not base timing, but total timing at idle, including the base). Just knowing that you have some drops to 14* shows that something is amiss. Assuming you have already tuned it up (plugs, fuel filter, etc.), I would suspect a dirty or failing fuel injector(s), coil(s) or IAC.

Is it giving you any trouble codes? How do all the plugs look?

I am not 100% certain, but that TPS reading of .39v just seems too low. That will lean out the idle mix. If the factory spec is closer to .50v, than .39v will affect the idle mix, which could be causing the miss at idle due to a lean mixture. Ed.
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Old 02-21-2005, 11:10 AM
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i have changed the spark plugs and wires, just to be sure, and please note that the TPS reading is in % not in volts. I have no idea how to translate that into anything more useful with the program.

The only codes thrown is if it is cold enough and running poorly enough you get a multiple misfire, which is understandable being that it is running so poorly.

Question the psi the MAP is giving would this be the same as a vacuum gauge would read?
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Old 02-21-2005, 11:45 AM
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The psi from the MAP is in absolute pressure, not gauge. To find gauge pressure, subtract your reading from atmospheric pressure (around 14.7 psi).
With your MAP at ~7 psi, that is -7.7 psig. Most vacuum gauges read in inches of mercury(in. Hg). 1 in. Hg = 0.5 psi, so with -7.7 psig you would have 15.4 in Hg vacuum. As edge said, this is ok.
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Old 02-21-2005, 05:32 PM
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Your inlet air temp never wavered and is off several degrees from your posted outside air temp. I suspect it is not working. Also, Edge mentioned the oscillating ignition timing. Besides jumping around, I think it is too retarded. Go back and check your base timing at 10 degrees. If that is good, check your distributor shaft wear, both up and down as well as side to side. I agree that the engine has not warmed up enough to go into closed loop. Last, low voltage to the system will cause some of your symptoms. Check your static battery voltage, then check that your alternator is putting out at least 14.2 volts.

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Old 02-21-2005, 06:01 PM
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Someone else has mentioned the voltage, so i've checked that and am getting 12.8 volts not running and 14.7 running. There is no distributor, 8 individual coils. I will run a longer data recording in the morning, it isn't supposed to be that cold (45 F) but i will record more.

[Edit]

I just went out and hooked up the computer (8:05 pm est) the truck is warmed up and the ambient temperature (according to the truck is 45 F) but the intake air temp registered at 77 F. Heat soak maybe, so i ran the rpms up to 3k and held it there for 30 seconds or so. 77 F steady. that would make the actual temp 40 degrees F or so. I'll get several data logs in the morning and see how much it changes.

What would be an acceptable descrepancy?

Last edited by tnoftsger; 02-21-2005 at 06:10 PM.
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Old 02-21-2005, 07:11 PM
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Coil packs and timing jumping around like that? Me know understand that, Kimo Saba. Like Edge says, after the engine has warmed, you should be looking at a steady 24 degrees until you change the load on the motor.

I can monitor my inlet air temp continuously, if desired, and it fluctuates a couple of degrees at idle and while driving. I have snorkeled fresh air, but always see an increase from cruise speed when slowing to a stop (normal because some hot engine bay influence) and a steady decrease when accelerating (higher air flow from demand and ram). I never see the big spread you have, but that could be because of engine bay air temp vice snorkeled air. However, I assume your hood is up when you are taking your logs, so the early logs should have been very close because your engine temp remained low. Your second log with warm engine and warmer outside air should have reflected a spread but there should have been some change during the runtime.

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Old 02-22-2005, 07:20 AM
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OK, here is this mornings data.

41-41 degrees F, hood up, first line of data engine not running (note RPM)

Also note I attempted to change the TPS sensor reading to Volts, it didn't work, so ignore those numbers.

The TPS is working when set to % open, ranged from 0% - 98% in later test with the engine not running.

I apoligize if the data is hard to read

Data Log
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Old 02-22-2005, 08:43 AM
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Faulty ground, i think theres some YamaGule in there

Have you checked the fuel pressure?

Have you checked the the coils individually with an ohm meter?

A rattle (timing chain, rod, wristpin etc.) can cause abnormal timing flucuactions as can bad gas(long periods of inactivity) or even the knock sensor.

To test the knock sensor take a light brass hammer and tap just beside the sensor while taking a reading. it should retard the timing. If it retards excessive then the sensor is too tight and if it retards erratically it may be bad.
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Old 02-22-2005, 10:17 AM
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fuel pressure is a rock steady 55 psi throughout. It runs pretty well when warm, horrible when cold. This morning it was just some rough idle. I haven't tried a hammer, but a .44 caliber one has crossed my mind.

I have checked the coils individually and found everything within spec.

It only has 68K miles, but i guess slop in the chain could happen by now.

at the end, when warm, the timing is only moving around about 5 degrees. I still suspect that the computer is moving the time around as a reaction to something.

If you'll notice the first advance spike (31 degrees @ ) happens just as the engine load peaks and the short term fuel mix kicks in.

The next peak at 100-106 seconds is at a short term adjustment peak

and the next at 136 is i don't know what and so on.

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Old 02-22-2005, 11:09 AM
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I'm going to go out on a limb here and say even with the low miles(probably many short easy trips?) that the intake valves are carboned up, absorbing fuel leading to a lean mix in the cylinders. I think it is babied around, maybe idles at stop lights for a majority of its life, never being wound out it has accumulated gunk and hasn't been able to shake it off. Many times if the carbon is excessive the heads must be disassembled.If not I'm stumped from here in cyberspace.

How many miles per day? What are the driving conditions( hwy, city, stop&go)? Does it smoke on startup? What percent throttle do you use to accel? How often is it wide open and for how long?
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Old 02-22-2005, 12:14 PM
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Your pretty close, Wife drives it most of the time. Average trip, 5 miles. Number of WOT, you'd be surprised, she has a lead foot.

I would say that carbon build up is unlikely, but so is everything else at this point. I've run some fuel system cleaners thru it, no change. But maybe something a little more aggressive might help.

Would a carbon situation like that only effect it when it's cold?

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Old 02-25-2005, 08:07 PM
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I dont think that the timing jump is a mechanical problem, at least in the ignition system. If it were, you would likely not see the jump in the log, but on a light only. I have to wonder is the ECU is going haywire. I just dont see anything that should be making the timing jump like that. It does not seem to correspond with anything from what I see.

Really weird.....


Chris
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