I have a 1999 Ford Taurus in which I recently performed a head gasket replacement on as well as a couple other parts - water pump, thermostat, etc.

EDIT: The car has a 3.0L V6 OHV Engine - Gas Only - Vin U for the curious

However, after completing this repair the engine has developed a surging or looping idle condition. Strangely enough though, there are no adverse or unusual effects when applying any throttle.

I have performed a series of checks including:

  • spraying for vacuum leaks
  • checking vacuum pressure
  • cleaning the throttle
  • checking the throttle position at idle
  • acceleration tests
  • cleaning the IAC
  • checking spark gap
  • performing Ford's recommended idle relearn procedure for this car
  • monitoring fuel trims
  • monitoring MAF data
  • monitoring O2 data

Even after all this I've been unable to track down the reason for this condition. The only hunches I have thus far are the following:

  • a bad IAC
  • a bad EGR or EGR tube
  • bad plugs

I would appreciate any thoughts as I'm pretty stumped at this point. Thanks in advance.

EDIT: I recently checked the hunches I had before and still came up empty. Swapped spark plugs with known good plugs but no change occurred. I swapped the IAC and tried cleaning it as well once again to no change. I removed the EGR and it seemed to function normally, it does hold a vacuum like it should and moves what I assume to be freely. I also cleaned it but nothing. I've also since tried cleaning the MAF and the intake air temp sensor, checking the intake air baffle for leaks, the plastic ridged tube after the air filter, and spraying around the injectors but alas nothing found.

Some other things I've noted during my search:

  • The computer seems to identify the idle issue as a misfire but I cannot tell if it's related to a singular cylinder or engine wide
  • Also I performed a cylinder power balance test by disconnecting the spark plug wires and I couldn't find a cylinder that didn't impact the idle
  • The long term fuel trims seem to oscillate with the surging of the engine which makes sense for a misfire
  • The O2 sensors do the same
  • One bank - not sure which - does vary more than the other at idle and has a consistent slight positive trim - though not enough to cause suspicion - less than 5%


  • Ben: If I disconnect the IAC the engine drops severely in RPMs shortly before stalling out. Also by charging voltage I'm assuming you mean volts during operation and while I haven't noted this specifically I believe they were around upper 13s last I looked

  • More info: Just more hunches and please correct me if I'm wrong but at this point I've almost entirely ruled out ignition as the causation of this issue. I've even swapped the coil pack with a known good one - forgot to mention this earlier. The only ignition possibility left is some gremlin or strange voltage issue or grounding issue. I've checked all my ground points and they seem in good condition, voltage is steady but small variations could be to small to detect with my scan tool.

  • What's left is either fuel delivery or some mechanical issue. I'm not crazily inclined to believe it mechanical because the car idled fine before and I didn't touch the cylinders and the only thing I did with the valves was replace the stem seals. While I admit it's possible it doesn't seem likely - again the car has power as it should and any throttle removes the issue completely.
  • So fuel delivery then. I did replace the injector seals and what I call the hats - a plastic bit that sits over the nozzle of the injector. I did this because the injectors are original and the other plastic bits would've broke apart if you sneezed on them - many broke upon removal of the injectors. The new "hats" were a bit different and could've possibly been more restrictive - especially if the injector's spray patterns were already messed up. No I haven't checked the injector pwm signal because I don't have the equipment or the knowledge to know what I'd be looking at.

As helpful or not helpful as this all is this is all that I have at this point. If desired I can post some graphs taken from my scan tool about certain bits of data and all but at this point the data this car could provide - remember 1999 - is either going to prove unrelated or unhelpful. Literally any ideas anyone has I'd be happy to investigate so please let me know if you have a thought. Again, thanks.

EDIT: Quick question, I noticed on someone else's post that they talked about timing advance - is there any way this may have some impact to the issue I'm having?

EDIT: So it's been about 5 months since my previous update to this question so I'l start with what i know currently.

  • Part of the issue was the fact that I had initially installed the injector pintle caps, what I called "hats" previous because i didn't know what they were called, incorrectly. These caps were not properly seated against the injectors causing the fuel spray pattern to be interrupted and therefore causing an extreme hunting condition.
  • Also, this issue is actual not related to the head gasket replacement as I originally thought. Originally, the car ran rough from what I assumed were bad spark plugs. This only turned out to be partially true as the engine still ran mildly rough following their replacement. The incorrect pintle cap installation only aggravated the situation moreso.
  • Another thing I have discovered is that this issue is not strictly related to idle. The engine has a strange tendency to suddenly surge lightly and even at highway speeds.

So what is the actual issue? Honestly still working on figuring it out but its looking more and more like old age. What I mean by this is that recently I've been diving deeper and deeper into this issue and testing every little piece of equipment that I can. The only positive impacts I've made thus far have been by swapping the coil pack and the EGR vacuum solenoid. Now, to be clear, I DID NOT swap these in blind hope or as a last ditch effort. According to ford specifications as well as information from a haynes repair manual. The coil pack should meet a certain ohm reading for both the primary and secondary windings of the pack itself. .5 Ohms per primary and 13500 ohms for secondary. The old coil pack did not meet this requirement and so I swapped in another coil pack from a 2000 year model taurus I have, its a parts car at this point. The 2000's pack was slightly closer to spec but still not within but I swapped it in anyway. The same was true for the EGR vacuum solenoid.

Unfortunately, I do not have solid quantitative data to prove this, however, the idle did seem to clear up slightly, although a slight hunting is still present.

At this point any recommendations or requests for info/data are still very much welcome but it is beginning to look like more of a case of needing a massive tune up. According to online sources the coil pack is apparently at the average age in which they go bad and I can reasonably assume the same about other similar pieces as the solenoid i mentioned earlier was also out of spec (electrically) and was leaking vacuum into the EGR valve.

In the mean time, I will be looking to replace the solenoid and coil pack with new units as they are most clearly bad and I will provide another update at that point.

  • Whats the charging voltage doing? What happens if you unplug the iac?
    – Ben
    Mar 4 '18 at 14:20
  • I have the same engine, but a '94. Does your model have a fuel regulator? It might be worth a check.
    – Carguy
    Mar 7 '18 at 7:44
  • Also, did you replace the fuel injector "hats" at the time of the head gasket replacement? Or, did you do so after, as a way to troubleshoot the surging idle?
    – Carguy
    Mar 7 '18 at 7:45
  • Yes, my engine does have a regulator and the "hats" were installed at the same time as the head gasket replacement due to the fact that half were already broken
    – Techlord
    Mar 7 '18 at 8:27

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