P1294: cleaned IAC, no obvious vacuum leaks, CCV elbow is intact, what's next? - JeepForum.com
 
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post #1 of 13 Old 12-29-2019, 12:35 PM Thread Starter
StellaTheZJ
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P1294: cleaned IAC, no obvious vacuum leaks, CCV elbow is intact, what's next?

I can't figure this out. I got a P1294 code and the engine has been taking a looooooong time to slow down between shifts. Further, it has been idling close to 1200RPMs a lot. That sure sounds like a dirty IAC, so I took of the TB and cleaned everything. This made no difference. I cleared the code, and it came back not too long after. I knew it would, because ti drove the same!

So I swapped in another IAC from a TB off a running truck. Not my truck so I don't know how it drove, but I hadn't heard any complaints. No difference.

I went through all the vacuum lines in the engine compartment, for the purge valve, for the heater, for the CCV, the brake booster, and cruise control. Maybe... maybe tow of the elbows on the intake manifold might have been a bit looser that I'd like (grasping at straws here), so I snugged them on tighter with a wee zap strap. Otherswise, I can't see of feel anything cracked or broken on any of these lines.

To be thorough, I took the rear CCV elbow off the valve cover to confirm that the plate with the orifice in it was still there. It was in perfect shape and nice and clean too, so it went back in.

Yes, I have searched here and on google for a long time to find something to check that isn't described above!! Surprising number of people have the elbows on the intake manifold fail... so I checked them by actually removing them and looking at them before reinstalling them!

This is a bone-stock engine except for newer injectors that have been in there for 3 or 4 years, and it only has 225,000km on it. What's next? What am I missing here?? Thanks for any ideas folks!

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post #2 of 13 Old 02-10-2020, 07:03 PM Thread Starter
StellaTheZJ
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Here's a bit of an update as a bump, and a cry for help too! Still pulling the same code...

I have replaced the IAC motor with another used one, and it acts about the same. In fact, I have noted that the RPMs sometimes drop to around 600RPM for a second or two, then creeps up again to idle close to 900rpm, with no AC or anything turned on. That information has about convinced me that BOTH IACs are working fine (they CAN control the idle to as low as 600RPM, but they DON'T). Hmmm..

I also noted that idles at 1200 if it's moving, even if it's only rolling a wee bit. If there's a signal from the Vehicle Speed Sensor, it commands the IAC to open more. That sucks! Is there something I can do to fix this?

It is not nice to shift (waiting for the revs, which never drop below 1200), and it's not nice to drive slowly (because the revs are always at 1200RPM or above when moving), and it's pulling the code over and over again. I KNOW I'm nissing something... any ideas??
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post #3 of 13 Old 02-10-2020, 07:29 PM
mukluk
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Your symptoms sound like you have a vacuum leak. What is your manifold vacuum reading at idle?

It's normal for the idle to be elevated with a closed throttle and a signal from the VSS, that's part of the PCM's normal programming for the idle speed. With a closed throttle and no VSS signal the PCM sets the idle speed via the IAC to a specific RPM value, but with a closed throttle and present VSS signal the PCM sets the IAC to maintain a specific MAP sensor value. Most often this higher idle speed while moving is closer to 1000rpm, so that lends credence to the theory your idle MAP reading is a bit low due to a vacuum leak.

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post #4 of 13 Old 02-10-2020, 09:33 PM Thread Starter
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I thought it sounded like a vacuum leak too. I looked all over the place at the wee lines for the cruise control and the heater, as well as the CCV hose. I also "sprayed" propane all around the engine while it was running, including on the innjectors, the throttle body gasket, the intake manifold gasket, and the vacuum fittings/connections on the side of the intake manifold. I couldn't cause any change in the idle speed by doing any of that, so if I do have a vacuum leak maybe it's inside the vacuum controls of the heater? But the heater works fine and I can't hear any hissing inside the cabin. Maybe the MAP sensor itself is wonky. I'll have to check the SnapON MODUS to see if I can do any tests on the MAP.
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post #5 of 13 Old 02-10-2020, 09:51 PM
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If you have access to another Throttle Position Sensor swap out yours and see what happens.

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post #6 of 13 Old 02-10-2020, 10:14 PM
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If you have a scanner with live data, watch the short term fuel trim for a response while you're checking for leaks at idle with propane or some other flammable medium (I prefer sensor-safe throttle body cleaner), that can pick up a change in mixture where the idle speed/quality isn't really noticeable. You can test your HVAC control leak theory by comparing the vacuum readings with the small takeoff line attached to the manifold vs with the line removed and the fitting capped.

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post #7 of 13 Old 02-11-2020, 10:45 PM
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Is this your 1996 ZJ?

You say the high idle happens when you are moving. Does that mean it idles OK when not moving?

From what I read it appears you have cruise control (CC). If so, try removing the CC cable from the TB. The CC is one place the speed sensor is in play (also vacuum actuated).
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post #8 of 13 Old 02-12-2020, 06:31 AM
jtec
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any relationship to this issue and installing "bored" TB?

Are there any EVAP codes or issues?

any other codes ANY?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #9 of 13 Old 02-12-2020, 05:50 PM Thread Starter
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Quote:
Originally Posted by Rubi4MyMrs View Post
Is this your 1996 ZJ?

You say the high idle happens when you are moving. Does that mean it idles OK when not moving?

From what I read it appears you have cruise control (CC). If so, try removing the CC cable from the TB. The CC is one place the speed sensor is in play (also vacuum actuated).
Oops, I forgot to mention this is on my 2004 TJ 4.0l. That said, the idle is high all the time, hence the code it's pulling. It is idling around 850RPM when still most of the time, but I guess it runs a bit higher every once in a while to trigger the code.
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post #10 of 13 Old 02-12-2020, 05:52 PM Thread Starter
StellaTheZJ
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Quote:
Originally Posted by jtec View Post
any relationship to this issue and installing "bored" TB?

Are there any EVAP codes or issues?

any other codes ANY?
This engine is completely stock, except for having newer 12-hole injectors, but they have been in there for about 4 years now, and the code is only showing up for the last month or two.

There are no other codes at all!
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post #11 of 13 Old 02-12-2020, 09:44 PM Thread Starter
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For anyone still following along with this, or who has the same problem, I learned of another test and I tried it out tonight. If you have access to a scanner, and you can read the Short Term Fuel Trim (STFT) values, and the Long Term Fuel Trim (LTFT) values, they can be useful diagnostic information. For instance, if you note that the LTFT values at idle are positive, like maybe +10% as an example, that means that the engine is adding more fuel than the factory programming expects. In itself, that might not mean much. But! If you rev the engine up and the LTFT goes a bunch closer to 0 (where the factory expected the fueling to be), that can indicate that you have a vacuum leak. You see, a vacuum leak makes a bigger percentage difference to the fuel ratio when not much air is entering the engine - which is at idle. On the other hand, at high RPM, there's so much more air going through the engine that a vacuum leak makes a much smaller contribution to the air fuel ratio, so there is less correction required by the PCM.

On the engine in this TJ, the LTFT values are about -6 or -7% at idle, and they rise to -8 to -9% at high RPMs. In other words, this engine is running RICHER than the factory expected at idle, and at high RPM. If it had a vacuum leak, it should be running leaner than idle. This is further confirmation that this TJ does NOT have a vacuum leak. I still don't know what changed in the Jeep to cause it to rev that wee bit higher and trigger the code!!

I hope this thread helps someone else out there!!
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post #12 of 13 Old 02-13-2020, 05:56 AM
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The issue has been high idle? If you had a vacuum leak would't the idle be lower and maybe the engine running a tad rougher, then when you spray it with the propane the idle would rise?

So wouldn't that pretty much rule out the vacuum leak? I would guess TPS.

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post #13 of 13 Old 02-13-2020, 07:17 PM Thread Starter
StellaTheZJ
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No, on a fuel injected vehidle a vacuum leak at idle means more air enters the engine... sot he O2 sensor reads that the exhaust is "Lean"... so the PCM adds more fuel to the mixture.

Due to the feedback loop based on O2 readings, adding or taking away air from the engine will be compensated with more or less gas too. So it runs great, just too fast for that throttle setting!
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