98 Explorer SOHC Rough Idle and Mid-speed Hesitation

[ThaaaCrusher]

Hello,

I've posted before about this same issue. X runs rough at idle, cold or hot. It also has an annoying hesitation around 35-40 MPH.

I've been dealing with it for years and not had any luck resolving. Here's what I've done to remedy the problems with suggestions for this forum:
Replaced Plugs/wires
Replaced Mass Airflow Sensor
Replaced Coilpack
Fuel Pressure checked - fine
Compression test - fine
EGR value replaced
Smoke-tested for leaks - a few small ones fixed

So I think I'm down to intake gaskets and O2 sensors. I'm coming to the point where I may just want to get rid of the truck. No mechanic has been able to figure it out. No codes being thrown.

The question is could the intake gasktes or O2 sensors cause the hesitation and the idle problem? I've read various things, but most seems to think tehy could cause the idle problem but not a bucking or hesitation problem. It also seems that most that experience the intake gasket problem only have rough idle when started but once the engine warms up, and tolerances narrow, the problem goes away.

Any way I can test to see if it's the gaskets or O2 sensors? I hate poke-and-hope, but with no codes being thrown and the typical failure of the gaskets, should I try it? I'd really like to test it first.

Any help would be appreciated.

 

[vroomzoomboom]

what motor is it. SOHC or OHV? SOHC's normally have a bad idle until it warms up, but will go away once its warm for the intake gaskets. the earlier OHV's would have a bad idle, and sometimes buck at speed because the upper intake gasket would get sucked in around the fuel rail. you can try spraying something like brake clean around the intake and see if the idle changes. or look for some dry rotted vacuum lines

 

[2000StreetRod]

smoke generator

The only reliable way to test for a vacuum leak is with a smoke generator. Using an aerosol is not very effective because access to places where there might be leaks is limited and the PCM quickly adjusts the idle speed to compensate for the aerosol by changing the spark advance and also the IAC valve setting.

Your 1998 SOHC V6 has the variable intake system (VIS). Have you checked to see if it functions correctly?

 

[ThaaaCrusher]

It's an SOHC

Ok, got it. Varaible Intake System (VIS). No, I don't know how to test it. Is there a thread that talks about this?

Also, do the symptoms of the rough idle and 35-45 MPH hesitation/jerking point to the possibility of bad O2 sensors? It's hard to tell from others who have posted about O2 sensor problems.

 

[2000StreetRod]

activates at 3000 rpm

The PCM is supposed to activate the VIS at 3,000 rpm. There's a vacuum switch, a vacuum motor and a lever that decreases the intake manifold runner length. Ford eliminated it on the V6 in 1999 probably because it wasn't very reliable and the typical Explorer purchaser was more interested in utility than performance. I was going to convert my 2000 to VIS but opted for a supercharger instead. Often the linkage between the vacuum motor and the lever comes loose.

It's more likely that your intake manifold gaskets are leaking than your O2 sensors are bad enough to cause noticeably poor performance.

 

[ThaaaCrusher]

intake gasket vs O2 sensor

Ok, so from what I've described with the rough idle and hesitation/bucking around 35-45 mph, you would think it's the intake gaskets more than anything else if I'm reading you right. Again, given my symptoms, no codes being thrown, performance shortcoming at similar RPMs, and your claim that it isn't an easy thing to test for would make it seem intake gaskets are the next thing to go for? Without the codes being thrown, I'm really not sure what to do but this seems to be such a common problem, maybe it's worth a shot?

 

[2000StreetRod]

check the VIS first

I suggest that you check the VIS first since it is a lot of work to replace the upper and lower intake manifold gaskets. The most common VIS related failure is the rod that links the vacuum motor to the lever has fallen off and is laying somewhere nearby. If your engine speed rarely exceeds 3,000 rpm then the lever could be stuck in the low rpm position due to carbon build up in the intake manifold.

If your engine revs excessively at cold start that is a good indication of leaking intake manifold gaskets. The cold shrinks the dried out gaskets causing more leakage. As the engine warms the gaskets expand reducing leakage. If the gaskets have never been replaced they are likely leaking by now. If you decide to replace the gaskets I suggest that you purchase a new O ring for the EGR tube because it will also be hard and possibly break when you remove the upper intake manifold.

It is a possible source of vacuum leak often overlooked.