BizarroStormy Posted October 26, 2020 Share Posted October 26, 2020 Quick sanity check before I pull my manifold apart. My truck recently developed the wonderful habit of increasing its idle speed as it comes up to temperature. Starting at around 900 RPM and ending up at around 3400 RPM when it gets up to operating temperature. After cooling off the cycle starts over again. I checked all the vacuum hoses and discovered my EGR was not properly hooked up and found that the green vacuum line to my transfer case was broken (fixed my always on part time light problem). I pulled my throttle body and cleaned it out, especially around the idle servo but it was not that bad to begin with. Re-installed with a gasket and then with RTV after it didn't do anything. I purchased a REM from Nick (Thanks nick this thing is awesome) and it looks like all my sensors are working and it runs in closed loop even when going crazy with the high idle. So.... This is a manifold leak right? Link to comment Share on other sites More sharing options...
omega_rugal Posted October 27, 2020 Share Posted October 27, 2020 disconnect the IAC... it may be opening but no closing... Link to comment Share on other sites More sharing options...
BizarroStormy Posted October 27, 2020 Author Share Posted October 27, 2020 I disconnected the IAC while it was cold and started the truck and it shot up to 2k rpm immediately. Shut down the truck, plugged the IAC back in and it went to 900 rpm. Unplugged the IAC while the truck was running and it slowly went up to 2k rpm after warming up. So it does seem to be IAC related. I replaced this IAC a few months ago to resolve another idle issue I was having so I assumed it would still be working. Should have a replacement in tomorrow. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 27, 2020 Share Posted October 27, 2020 Want to get a MoPar IAC and be done with that sensor for a while? Link to comment Share on other sites More sharing options...
fiatslug87 Posted October 27, 2020 Share Posted October 27, 2020 What's the Mopar PN for the Renix 4.0L IAC? Found it 83503643, don't know if there's a superceded number for it. I'm interesred in one. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 27, 2020 Share Posted October 27, 2020 That’s the correct part number. Kinda a pretty penny for one too. Link to comment Share on other sites More sharing options...
omega_rugal Posted October 27, 2020 Share Posted October 27, 2020 you can clean that IAC just in case dirt is casuing the trouble Link to comment Share on other sites More sharing options...
BizarroStormy Posted October 28, 2020 Author Share Posted October 28, 2020 Tried the new IAC today and same result. Any other ideas? Link to comment Share on other sites More sharing options...
cruiser54 Posted October 29, 2020 Share Posted October 29, 2020 3 hours ago, BizarroStormy said: Tried the new IAC today and same result. Any other ideas? Yes. Go to www.cruiser54.com and complete Tips 1 through 5. Make sure your intake manifold bolts are tight. Link to comment Share on other sites More sharing options...
omega_rugal Posted October 29, 2020 Share Posted October 29, 2020 4 hours ago, BizarroStormy said: Tried the new IAC today and same result. told ya Link to comment Share on other sites More sharing options...
omega_rugal Posted October 29, 2020 Share Posted October 29, 2020 1 hour ago, cruiser54 said: Make sure your intake manifold bolts are tight. wait a sec. a vacuum leak with the throttle closed and low rpms makes the ECU inject more fuel? because just letting extra air in would lean the mixture not make the engine rev up Link to comment Share on other sites More sharing options...
cruiser54 Posted October 29, 2020 Share Posted October 29, 2020 Just making sure they're snug. Link to comment Share on other sites More sharing options...
scguy Posted October 29, 2020 Share Posted October 29, 2020 wait a sec. a vacuum leak with the throttle closed and low rpms makes the ECU inject more fuel? because just letting extra air in would lean the mixture not make the engine rev upUh, no. How do you think an IAC or throttle body affects the idle? By letting more air in. Vacuum leaks can and will raise your idle. Link to comment Share on other sites More sharing options...
omega_rugal Posted October 29, 2020 Share Posted October 29, 2020 7 hours ago, scguy said: How do you think an IAC or throttle body affects the idle? by letting more air in AND injecting the corresponding amount of fuel... Link to comment Share on other sites More sharing options...
scguy Posted October 29, 2020 Share Posted October 29, 2020 by letting more air in AND injecting the corresponding amount of fuel...Which the oxygen sensor provides/commands when it senses a lean mixture. Link to comment Share on other sites More sharing options...
omega_rugal Posted October 29, 2020 Share Posted October 29, 2020 Just now, scguy said: 5 minutes ago, omega_rugal said: by letting more air in AND injecting the corresponding amount of fuel... Which the oxygen sensor provides when it senses a lean mixture. yes, and the ECU is aware of this, in case of a vacuum leak, the ECU doesnt know, or is too dumb to let more fuel in even if the IAC is closed more air? ok there goes more fuel.... wait a minute, i didnt let that air in... Link to comment Share on other sites More sharing options...
scguy Posted October 29, 2020 Share Posted October 29, 2020 The ECU isn't dumb. It can see a leaner mixture and adjusts the fuel trims to keep the engine running at an appropriate air/fuel mixture. Go unhook a vacuum line on any gasoline fueled running engine. It will rev up, unless it's so large of a vacuum leak that the computer can't compensate, in which case it will run poorly or stall. Link to comment Share on other sites More sharing options...
omega_rugal Posted October 29, 2020 Share Posted October 29, 2020 7 minutes ago, scguy said: The ECU isn't dumb. It can see a leaner mixture and adjusts the fuel trims to keep the engine running at an appropriate air/fuel mixture. not dumb but ignores the engine revving up even with the throttle closed Link to comment Share on other sites More sharing options...
scguy Posted October 29, 2020 Share Posted October 29, 2020 not dumb but ignores the engine revving up even with the throttle closedIt will try to close the IAC to bring the idle back down, but if the leak is substantial (more than closing the IAC can handle), there's nothing it can do. Link to comment Share on other sites More sharing options...
derf Posted October 29, 2020 Share Posted October 29, 2020 The 4.0 is a speed-density system. It injects fuel primarily based on the speed of the engine and the density of the air inside the manifold. The O2 sensor just adjusts trim. If you open up a vacuum hose, the vacuum in the plenum will drop. That will cause the engine to react by injecting more fuel. It may try to compensate by closing the IAC but you can only close it all the way. Once the IAC is fully closed, it can only compensate by adding fuel, raising the idle. Link to comment Share on other sites More sharing options...
BizarroStormy Posted October 29, 2020 Author Share Posted October 29, 2020 20 hours ago, cruiser54 said: Yes. Go to www.cruiser54.com and complete Tips 1 through 5. Make sure your intake manifold bolts are tight. We have a winner! The rear bolts/nuts near the fire wall weren't even finger tight! So everyone saying manifold/vacuum leak = more fuel = higher RPM are correct. I could even see the ECU reporting the O2 sensor going lean/rich as the idle increased. So nice to have my truck back, time to go fishing! Thanks everyone for the advise. Link to comment Share on other sites More sharing options...
cruiser54 Posted October 29, 2020 Share Posted October 29, 2020 48 minutes ago, BizarroStormy said: We have a winner! The rear bolts/nuts near the fire wall weren't even finger tight! So everyone saying manifold/vacuum leak = more fuel = higher RPM are correct. I could even see the ECU reporting the O2 sensor going lean/rich as the idle increased. So nice to have my truck back, time to go fishing! Thanks everyone for the advise. what about the Tips? Link to comment Share on other sites More sharing options...
BizarroStormy Posted October 30, 2020 Author Share Posted October 30, 2020 I did most of 1-5 previously. I haven't checked all the grounds but everything seems to work now. Link to comment Share on other sites More sharing options...
cruiser54 Posted October 30, 2020 Share Posted October 30, 2020 For now........ Link to comment Share on other sites More sharing options...
CoastChief Posted November 2, 2020 Share Posted November 2, 2020 I have this same problem. gonna give it a try tomorrow morning. Do you have any photos of the intake manifold? Link to comment Share on other sites More sharing options...
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