high (5krpm) idle after cylinder head replacement (4.0L 99 WJ) - JeepForum.com
 
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post #1 of 14 Old 01-13-2020, 10:29 PM Thread Starter
breadboard
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high (5krpm) idle after cylinder head replacement (4.0L 99 WJ)

Hi Jeep People,
I've been slowly going through the process of replacing cylinder head with a refurbished one due to valve tap noise on the original one (250k+ miles!) and now everything is mostly back on. I didn't put the plastic hood on the throttle body yet. Went to start it up and the engine ramps up to about 5k RPM and then flutters around there. Initially I thought the air bypass valve went bad (the jeep has been sitting there for ~3 months now) so that was replaced with same result. Verified the three vacuum lines on the side of the intake manifold are in place, also verified PCV feed into the intake manifold is in place. the engine does not seem to misfire so I'm guessing the crank position sensor is working. If I turn the car to run, but not start the engine, just the check gauges light comes on (not check engine) and i get one beep, followed by three beeps, followed by three more beeps. I replaced all top engine gaskets and torqued bolts to spec, but is it possible I seated intake manifold incorrectly and there is a gap? I was thinking of clamping on a plate with gasket material over the throttle body inlet to see if engine starts (there is a vacuum leak) next. Was curious if forum folks had any other troubleshooting advice. Many thanks.

P.S. I'm no jeep expert but OK at following directions.

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post #2 of 14 Old 01-14-2020, 05:04 PM
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I had a high idle (about 2000 rpm) after pulling/replacing the manifolds on my 4.0L ZJ to fix some leaky core plugs. Turned out to be the new gasket not sealing even when everything was torqued to spec (and as far beyond as I dared), so I went for a Remflex gasket which sealed perfectly at factory torque spec.
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post #3 of 14 Old 01-14-2020, 06:30 PM
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5000 rpm is a bit more than a high idle!
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post #4 of 14 Old 01-14-2020, 10:10 PM Thread Starter
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Quick update!
I plugged up (not connected, plugged) the three vacuum line ports on the side and two on top. Also I took throttle body off and manually extended the IAC so it is closed by using 12V source and alternating polarity on outer pin pair. I then also verified there was no debris in the throttle body keeping the throttle open (I disconnected linkage). With IAC unplugged electrically, fired up the Jeep. Sure enough, it took a little longer to start but then went to high RPM again. Took throttle body off, inspected IAC, it was still fully closed. Now I suspect the aftermarket gasket (which is thicker than the metal MOPAR gasket) is seated improperly, or the intake manifold is seated improperly. In any case, I think the intake manifold needs to come out next for inspection.
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post #5 of 14 Old 01-14-2020, 11:48 PM Thread Starter
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One more update, running out of good ideas and moving onto dumb ideas, I decided to test hypothesis that my intake manifold had a leak. I took off the throttle body and sealed the intake hole with a piece of metal that had gasket material on it. Sure enough, the engine would crank but not start so vacuum leak around intake manifold or the ports I plugged up is not likely. So if the IAC is fully closed and electrically disconnected (so it cannot be opened), what can it be? Re-reading the FSM, it looks like the automatic transmission throttle linkage needs to be re-adjusted after cylinder head replacement. Not only did I not re-adjust mine, but the clip is disengaged so that linkage is loose. I am now thinking that the engine generates enough vacuum to open the throttle valve, and since the automatic transmission cable is not providing counter force, it just keeps going. Will adjust it tomorrow and see what happens.
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post #6 of 14 Old 01-16-2020, 10:21 PM Thread Starter
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Did some more investigation and am now dumbfounded. If I take off the throttle body and seal off the port on the intake manifold with a block of metal with soft gasket material then the engine doesn't start (good!). If I install the throttle body with IAC installed and cover the inlet to that with the same block, the engine starts up and goes to 5k rpm. I replaced the throttle body to intake manifold gasket with something from a thicker rubber (cut myself) and tightened it down. I also covered up the IAC port completely with soft gasket and metal back, thereby sealing it. So now with a thicker gasket, IAC fully sealed, top of the throttle body sealed, engine still starts and goes to 5k rpm. I'm going to cut a plate to pull vacuum from the bottom of throttle body and use a smoke pen to see if I can detect a leak.


Question for Jeep experts: Has anyone ever seen this much air get into throttle body along the shaft or through MAP sensor or through TPS sensor? Any hints as to what to test or should I just look for rebuilt throttle body?

Many thanks.
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post #7 of 14 Old 01-17-2020, 05:40 PM
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Quote:
Originally Posted by breadboard View Post
Did some more investigation and am now dumbfounded. If I take off the throttle body and seal off the port on the intake manifold with a block of metal with soft gasket material then the engine doesn't start (good!). If I install the throttle body with IAC installed and cover the inlet to that with the same block, the engine starts up and goes to 5k rpm. I replaced the throttle body to intake manifold gasket with something from a thicker rubber (cut myself) and tightened it down. I also covered up the IAC port completely with soft gasket and metal back, thereby sealing it. So now with a thicker gasket, IAC fully sealed, top of the throttle body sealed, engine still starts and goes to 5k rpm. I'm going to cut a plate to pull vacuum from the bottom of throttle body and use a smoke pen to see if I can detect a leak.


Question for Jeep experts: Has anyone ever seen this much air get into throttle body along the shaft or through MAP sensor or through TPS sensor? Any hints as to what to test or should I just look for rebuilt throttle body?

Many thanks.
I'm no jeep expert and am not a mechanic. Just someone who has done all my own car repairs for 40+ years. I'd put it all back together and check for an intake gasket leak by spraying carb cleaner all along the edge of the intake and listening for a change. Then similary check all other possible sources of vacuum leaks - including vacuum lines and the possibilities you mentioned.
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post #8 of 14 Old 01-18-2020, 07:30 PM Thread Starter
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Originally Posted by pholverson View Post
I'm no jeep expert and am not a mechanic. Just someone who has done all my own car repairs for 40+ years. I'd put it all back together and check for an intake gasket leak by spraying carb cleaner all along the edge of the intake and listening for a change. Then similary check all other possible sources of vacuum leaks - including vacuum lines and the possibilities you mentioned.
I will give this a shot, thanks for the advice. If I cover the port on the intake manifold (with vacuum lines attached except PCV and brake booster plugged) then the jeep doesn't start, for this reason I don't think the issue is the lines. If I install the throttle body and cover the top then I get to 5k rpm. I agree that the gasket is a suspect. I don't have a second person handy to run the jeep while I'm in the engine bay so I'm going to try to cut a gasket out of 1/4" material and try again to see if any difference. I also set up a rig and confirmed the throttle body can hold -0.5 bar with both ends covered so I think that is pretty good.

(I'm to new to post links, so can't link, but hopefully soon!)
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post #9 of 14 Old 01-19-2020, 06:55 AM
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Install TB, remove all linkage.
Remove and plug the Vac lines, Brake booster, EVAP, and HVAC, PCV. Did I miss anything?
Have you taken the time to check codes?
Intake manifold leak is first guess - any whistling sound exhaust noise?
The suggested carb cleaner spray trick is fast easy.


Start engine - what is idle RPM, Any new codes appear?
Why not purchase a new TB gasket as opposed to fabricating?

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 #10 of 14 Old 01-19-2020, 06:59 AM
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You replced the cylinder head to solve a valve tap noise?
Did you check lifter, rockers, pushrods?

Something seams missing in cause and repair.
BUT idle issue is the current issue

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 #11 of 14 Old 01-19-2020, 12:58 PM Thread Starter
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Hi jtec, no apparent codes and unclear if I solved the noise issue since it's hard to tell when it's idling at 5k. I did some more debugging and realized that I made a mistake before: when I started up the jeep with intake manifold plugged, I did not have throttle body connected to computer was not getting MAP or TPS data. I'm now thinking that it didn't start due to lack of fuel and not lack of air in that configuration. I tried it again last night where i installed the plate on the intake manifold but left TB attached and Jeep started up and went to 5k. I now think that I did not seat intake manifold correctly and that's where the leak is coming from. I'm going to remove it today/tomorrow and inspect imprint on gasket or see if I can tell visually otherwise. Will keep group posted!
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post #12 of 14 Old 01-19-2020, 06:20 PM Thread Starter
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I think I found the issue! When I was pulling off the intake manifold, I used a borescope to make sure my socket was over the bolts and noticed that the two studs on the cylinder head were not aligned with the alignment features on the intake manifold, oops! Once I pulled of the manifold, I also saw marks on the metal where I clearly torqued against those studs. Imprint on the gasket (not MLS like OEM) is not uniform, where the top part was seated but the bottom was not. Looks like there was a gap in the bottom of the intake manifold and that is where the air was getting in. Will re-install the intake manifold, taking extra care to make sure things are aligned, this week and will report back. Some pics are here: https://ca.routed.net/jeep.
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post #13 of 14 Old 01-22-2020, 06:19 AM Thread Starter
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Solution found! I re-seated intake manifold carefully making sure the alignment features line up using a borescope, torqued thing down, and now the engine idles correctly. I think there is still a faint tapping noise (engine has 250k miles on it) so will debug that further later. At least it looks like vacuum leak was fixed. Thank you forum for pointers / info.
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post #14 of 14 Old 01-22-2020, 01:24 PM
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Glad you got it fixed! Thanks for letting us know what it was.

'00LMTD-4.7l,242hd,D30,D44A,3.73's,True-trac front,Spartan rear,4" short arm lift,JK'sW/32's
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