rough idle during open loop

[robs]

02 Rodeo sport 3.2 DOHC 106K

I'm getting a rough start in the morning. The truck starts and idles around 1200. For about a minute or so the idle surges to 1500 and back down. After this it idles normally around 800 and is fine for the rest of the day. Outside temp is around 70 morning 90 afternoon.

Here is what I have so far
- No stalling
- No check engine light
- Idles fine after a minute or so and for the rest of the day
- don't have codes


From reading the forums here I think this vehicle runs in open loop mode after startup. Does this rule out O2 sensors? I read about the IMG gasket but not sure how to tell if it's bad or not. Any others suggestions on what to look for?

Thanks in advance

[psguardian]

Easy intake leak check:
Have someone hold idle @ 1500rpm (after warm up), spray entire intake & vacuum system & hoses with 'o2 sensor safe' carb cleaner one area at a time. If there is a leak the non-combustable cleaner will get in & cause the engine to stumble/lose rpm. If nothing leaks, you've just cleaned your engine!

~psguardian

[robs]

just got back to working this issue. I sprayed around the intake manifold gaskets and saw no signs of a leak. Here is what I was able to narrow it down to.

Problem only occurs for the first 2 or 3 minutes when cold in the morning, with the AC on, and the truck in park.

Any other time during the day the idle is fine. If the AC is off the idle is fine. If I start the truck and immediately put it into park the idle is fine. If I shift back into neutral before it fully warms the idle fluctuates again.

What controls the idle under these conditions. computer, vacuum line, sensor?

Thanks
-Rob

[psguardian]

Someone correct me if I'm wrong.

On cold start (anytime) idle should run higher to facilitate engine warmup, dropping within 3-5min.
A/C forces higher idle to compensate for it's amp draw on batt & it's drag load on main pulley.
Shifting through any driven gear will cause idle flux due to trans momentarily putting a slight load on engine via torque converter engaging.

Issues aside these three conditions being placed on a 100k+ engine all at once should confuse & strain it but good.

Is this a new phenomenon or no?

~psguardian

[robs]

After rereading my initial post I think I should clarify. Sorry for the confusion, I didn't word the issue properly

The idle actually fluctuates when this happens. 1500 down to 1200 back to 1500 and so on for a minute or two. After that the idle steady back to normal. It's like the computer is trying to raise the idle to compensate for AC load and stuff but something is forcing the idle to drop back down

The problem started occurring right after I swapped the ac compressor and fan clutch. Both were broken for a while and essentially no load on the motor. Once everything was in proper working order, the issue cropped up.


Thanks
-Rob

[robs]

I think I narrowed this down to fuel pressure but stuck and really need help where to go next

So the problem seems to be getting a bit worse. I figured out if I first let the fuel pump prime by turning the key to accessory, the idle fluctuations are much much less (100-200RPM) but it never sounds like it wants to stall and never dips below 1200.

If I don't prime then the idle fluctuates down to 1K and back and it sounds like it might stall but doesn't

I thinking fuel pump or maybe the pressure regulator? I changed the filter but that didn't change anything. But why only in the morning and not *ANY* other time during the day? Acceleration is smooth and idle is fine any other time so fuel delivery seems fine then. Any other tests I can try? I see a what looks like a schraeder valve on the fuel rail on the drivers side. Can I just stick a gauge on this thing and see what my pressure is?

[Yotamonk]

Yes that port on the fuel rail is for connecting a pressure gauge. I'm wondering how long it takes between surges. Is it something like less than a second between 1500 rpm and 1200rpm, or more like 5 seconds between? If it's the latter, I would think it could have something to do with the A/C compressor turning on and off on it's own maybe due to being a little too low on oil/refrigerant. If it's short and erratic I would make sure the fuel pressure regulator is holding. I'm not familiar with rodeos but if it has a vacuum line connected to it, It should be dry with no fuel in the vacuum line. If there's any fuel there it's leaking and should be replaced.

[Buster28]

On a 02 Rodeo the fuel pump will run for 2 seconds when the ignition switch is placed in the Run position, when the ECM receives the 58X signal from the Crankshaft Position Sensor indicating the engine is being cranked the pump restarts. It will remain running as long as the 58X signal is received. If you attach a fuel gage to the test port and turn the ignition to Run and Off the gage should read about 38 PSI and should hold this pressure for at least an hour. When the engine is running at 1200 rpm the pressure will be about 34-35 PSI.

[robs]

Yotamonk

When I don't prime the surge is short maybe a sec and does seem to follow the ac compressor turning on and off. When I prime first, the ac compressor doesn't cycle and the surge is very small in comparison. I was thinking ac at first, but it's new and just recharged so level is fine. My theory is the motor goes to almost stall and the ecm turns off the compressor to reduce load which keeps the motor from actually dying.

buster
So I'm going to go find a guage to hook and and take some measurements. I read somewhere about the FPR leaking overnight or sitting for long periods causing the pressure to drop hence needing to be reprimed. So does this sound right

turn truck to run and pressure should be ~38psi.
turn off and watch it hold for an hour
turn on and rev to 1200 and pressure should be ~34/35

Any idea what a good pressure should be after the truck sits overnight before re priming it? How much pressure loss is considered normal?

[BigSwede]

Vacuum leak, such as from a bad IMG, would not manifest symptoms until closed loop operation. In open loop the computer runs off of a pre program, ignoring input from sensors.

[Yotamonk]

That procedure seems correct robs, after sitting over night the pressure would most likely be zero. Another thing to consider is the Idle Air Control valve. I believe these are a common thing to go on these and can cause symptoms like you're having.

[Buster28]

Every time you place the ignition in Run the fuel pump starts for 2 seconds, therefore the injectors are primed and operate as soon as the engine is cranked. If the pressure leaks down over night due to an internal or external leak, the engine will still start after several seconds of cranking. You can observe this by pulling the fuel pump fuse while the engine is running. The engine will stall in approximately 20 seconds. Replace the fuse and attempt restart, the engine will run on fuel after 10 seconds (more or less) of cranking. Btw, you should pull the fuse on the fuel pump and stall the engine before removing your fuel test gage.

[robs]

thanks guys so much good info here. Need to have a few beers and digest it all

Any idea where to find some gauges? Would something like this work ok
http://shop.advanceautoparts.com/webapp ... P2107_____

or this
http://www.harborfreight.com/fuel-injec ... 92699.html


No IAC on this. All drive by wire. Something about the ecm setting the tps to open more when idle and AC on. I need to read more about how that works to be sure.

I hear the fuel pump priming for 2 secs so it does seem to be working. I never have to crank this thing at any time to get it to start. First crank and she's running. When i changed the filter I did remove the cap and fuse. She did die on me real quick though something like a sec or two. Not sure that means anything

Thanks for the tip about relieving pressure when installing the gauge. Didn't even think about that. Hopefully have results this weekend.