Idle Issue

[gtyates]

Well truck is back in shop today. They ran a complete diagnosis and still nothing shows up as an issue. All sensors are functioning properly. Also checked fuel pressure and it is holding great. Even checked after truck was off and parked and was still holding pressure in the 30 pound range. Right now they are stumped as to what is causing this issue. They did mention that since I replaced the MAF sensor that the O2 sensors that were reading a little low are now reading correct. Could the throttle cable itself be causing these dropouts in rpm's? Of course that would not explain the rough start when engine is warm. I love the truck, but this is frustrating!!

 

[e_c_driver]

I had the surge in mine also when i let off the accelerator as if it was slowing itself down, I unhooked the cruise control cable at the throttle and it stopped. No cruise now but it will be fine until i fix the problem. mine still idles rough after a few seconds of sitting at a light and i have replaced everything..

 

[chandler1981]

gtyates,

Just checking to see if you found out anything yet. I am having a very similar problem but mine is 1997 4.0 SOHC. I am running a bunch of tests tonight and tomorrow so I will let you know if I am able to solve the issue.

 

[gtyates]

I have not resolved the issue on mine. Though yours is a SOHC amd mine an OHV it still could be a similar issue.

 

[e_c_driver]

I hear a loud hissing like a big vacuum leak but i cant find it. im sure thats whats causing the problem. Maybe the lower intake gasket,, Thats the only one i havent replaced... Soon to come tho.

 

[gtyates]

EC driver, do you have the CEL on? I do not, which is what is making this such a challenge as my computer does not think anything is wrong.

 

[gtyates]

Well, still no resolution to the issue. One more thing I am noticing, which keeps me thinking it is the TPS going bad, is that I seem to have spots in the rpm band as I am pressing the accelerator that are "dead". As the rpm's climb it seems it will hit areas where there is a pause in the climb as I am pressing the accelerator. Also seems to be a slight hesitation when I press the accelerator before it reacts. What is still puzzling also is the rough start-up when engine is warm. I have got to get me a multi-meter and test this.

 

[gtyates]

Another thought that comes to mind is the Air Intake Temperature sensor. Anyone have an issue with this going bad and causing odd drivability issues?

 

[gtyates]

Update. Just replaced the air intake temp sensor. Pulled negative battery cable, replaced sensor, and took truck on a drive. Seems to have improved significantly!!! Not ready to say it is cured, but much better after just one drive cycle. Still got a slight drop-off when lifting off the accelerator after topping a small hill, maybe 200 rpm's below where it actually settled and held steady to, but so slight I only noticed because I was staring at the tach. Also it did not have the surge in it after the rpm's dropped, it just popped back up and held steady. Just sitting in the driveway, revving to 3500 and then taking foot off accelerator produced a smooth drop in rpm's all the way to idle at around 900 on tach. Still may go ahead and replace the tps as well, but will wait and see how this does. I will add the factory sensor I pulled out had a red tip on the sensor wires that actually looked black almost like it had gotten hot and burnt the red colored coating.

 

[greasemanicure]

Since you already replaced the IAC, this is not likely to be it. On our Ex, the idle was rough but not throwing codes. Checked all the usual suspects, cleaned the MAF, IAC and changed the fuel filter. On a whim I pulled the black "cap" off the IAC, maybe it is a filter? and the idle evened out. Put the cap back on. Idle went to crap. Soaking did no good. Put on a dense foam bubbler from the fish tank for now. Is that cap something that can be replaced or do I have to change the whole IAC?

 

[gtyates]

I believe you will have to replace the IAC. I have never had any real luck cleaning one anyway. I bought mine from rockauto.com, a site vendor. I paid a little more and bought the Motorcraft one. As a matter of fact, the MAF and the IAT sensor were also Motorcraft. I figure the MAF and IAT lasted 14 years, so why not spend a little extra to replace them. lol!!

 

[alohamonte]

Have you replaced the ignition coil? I had a 93 caddy that had poor idle and would ocasionally stall out. I swore it was a vacuum leak even though one wasnt detected. No codes, no CEL. A shop replaced the distributor and it was magic - ran like new.

 

[gtyates]

Have not done the coil pack yet. Diagnostics did not detect an issue when they hooked it up for me. I am feeling confident that the IAT sensor has solved the vast majority of my problem. Still may replace the TPS. I need to get a multi-meter to help trace these things down!! If something is slightly out of spec but not enough for the computer to store a code it is a pain to find it!! Just about any shop you go to will hook it up for diagnostics and if nothing shows then as far as they know nothing is wrong. I like the guys I normally use, but they were stumped.

 

[alohamonte]

After reading this thread, I cleaned the air charge temperature sensor (there was plenty dirt and grim on it). Idle is a bit smoother, so thanks for the tip!

When I had idle problems, I just starting replacing stuff.... probably not the wisest choice.

I replaced the MAF, IAC, TPS, upper intake gaskets (and cleaned injectors), PCV, EVAP Pressure Sensor and EVAP Control Solenoid, and plugs & wires.

The EVAP parts were toast, the wires were degraded, the upper intake gaskets were leaking, the injectors were clogged, and the PCV elbow was blocked. For me, there was no silver bullet. The MAF/IAC/TPS were probably okay, but I replaced them anyway just to rule it out and because they were original.

 

[gtyates]

Another follow-up. After replacing the IAT sensor truck does run better and gas mileage has improved. But, am still getting the RPM surge in the 1100-1800 rpm range, most noticible on deceleration, but I also notice on acceleration it is not smooth in this range. Also still have the hot start-up issue where it runs rough on a hot engine start-up. I hate to just keep replacing parts for the fun of it. Still no CEL so no codes are stored in the system. As I watch this forum I see others are making very similar threads to mine, so I am not alone in this issue. lol!! Repairs so far are the MAF, IAC, and IAT. Previous repairs last fall include upper intake gaskets, camshaft position sensor, fuel filter, plugs & wires. Also have had the throttle body cleaned and fuel injector service performed. I am second owner, my Dad bought it new. Truck has 110k on it now, very low for a '96 model. Personally I am thinking the TPS and/or the coolant temp sensor as a possibility.

 

[gtyates]

Dropped truck back off at the shop. Still have the rpm surge and rough start when engine is warm issues. Based on feedback from this site I asked them to focus on the TPS and the Coolant temp sensor when scanning the system tomorrow. Will see what response I get from the shop. WIll update list if any resolution is found to this phantom rpm surge issue.

 

[gtyates]

Had truck back in shop today. They ran another full diagnostic test. Only thing showing up at all is 1 o2 sensor opening slightly slow, but in spec. Tomorrow they are doing a smoke test to look for any possible vacuum leaks, though nothing in diagnostics shows this. Frustrating!!!

 

[bradweezy]

take your pdfe sensor off see if theres moisture in it. my 95 did that before the sensor took a sh*t.about a week or 2 then check engine light came on yup pdfe,dead

 

[Nukeya]

I've got the same rough idle as you when warmed up. Can't seem to resolve it, I've thrown tons of parts and money at it still not resolved. One thing you mentioned was that you or one of the posters heard a loud hissing sound and that the brakes were spongy, could you have a vacuum leak in the power brake vacuum booster? Just an idea. I'm leaning toward bad egr or dpfe on mine, but could be vacuum too although I hear no leaks and can't find any, I'm as frustrated as you are.

 

[bradweezy]

vaccum solution

vaccum is easy just spay starter fluid around your vaccum line connetions, if the idol changes theres your leak. my original pdfe sensor went at about 115,000 miles and then again at 135,000. they say its all about the condensation forming in the exhaust, so i'm gonna try some gas treatment(to make sure theres no water in the fuel system)and will be doing a pdfe swap(made sure i bought 1 with a good warrenty),also the vac.hoses. if this works i'll post it:biggthump