1999 Explorer -Intermittent rough idle, 4.0 L SOHC


Asked by Feb 02, 2016 at 09:28 AM about the 1999 Ford Explorer 4 Dr XL 4WD SUV

Question type: Maintenance & Repair

I have no Check Engine light or codes. During idle it
will every few minutes start to stumble as if it is
going to stall. I am unfamiliar with this Ford 4.0L
SOHC engine (not to be confused with the 4.0L
OHV engine) and the common issues that it may
have. It accelerates well and seems to have the
power it should have. I ran a live Elm327 scan and
it appears that an injector was being stuck open at
times but i do not have enhanced DTC's for this
vehicle to see anything more in depth. However, I
am also thinking that a possible cause may be a
vacuum leak coming from the intake manifold. I
am not quite sure why there is no code that is
being set in the PCM. Hence why I am leaning
towards a possible intake manifold leak. Are there
any mechanics that are experienced with this
engine that can give me some input? Thanks in

6 Answers

have you checked: - Vacuum leaks - Fuel filter - Spark plugs and Wires (to check wires, just spray mist over top of wires if see any sparks, change them) _ Fuel injectors (clogged (needs replacement)/dirty) - IAC Valve/PCV Valve - MAF or MAP sensor - EGR Valve - Oxygen sensors (upstream and downstream, you might have 2 or 4 O2 sensors)

Best Answer

-Vacuum leaks, Not checked -Fuel Filter, Checked (fuel pressure good) -Wires and plugs new -Injectors, Cyl 5 sticky -IAC, EGR and Map, untested -MAF cleaned but untested -o2 sensors, voltage output good on each -TPS, not tested Any common weak spots for vacuum leaks on these engines?


As far as the sticky injector goes, I will just clean, change filters, and test spray patterns and replace as needed.

i use a smoke machine to find vacuum leaks..


I cleaned the IAC which basically was already very clean, as is everything under the hood, fortunately. Judging from the various internet search results and posts, it appears to be a very common issue to have a vacuum leak at the intake manifold upper plenum. The other being faulty/clogged fuel injectors. Both of which I will repair soon. I will post my results which I hope will help others if they have a similar problem.


Update: I replaced the upper and lower intake manifold gaskets. I am still having the goofy idle. I checked the fuel pressure, which was at a stable 63psi. I changed the fuel filter and now I am running at 64psi fuel pressure. I was listening to the engine idle and I heard an injector stick and the engine stumble. The issue is only getting worse as the vehicle sometimes has hard start issues. I checked the fuel pressure leak down and it immediately dropped from 64psi to 0 after about 15 seconds. The fuel pressure regulator is not losing any fuel to the vacuum line. I will be cleaning or replacing injectors soon.

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