irregular idle when engine warms up
 

I just bought my first jeep! a 98 4.0 tj. when the engine warms up it
"hiccups" at idle or when running at low rpm not under load. 81k miles,
new air filter, spark plugs, plug wires look good. any suggestions?

"Robert Coffey" > wrote in message
news:ghEme.9935$3u3.9116@trnddc07...
> I just bought my first jeep! a 98 4.0 tj. when the engine warms up it
> "hiccups" at idle or when running at low rpm not under load. 81k miles,
> new air filter, spark plugs, plug wires look good. any suggestions?
>

Did it do this before the tune-up? We need to know whether
something in the tune-up did this, or not.

Might be time for an O2 sensor (or two...).

Have you checked the plug wires with an
ohmmeter? How's the cap and rotor?
Vacuum advance working? Timing set?
No vacuum leaks?

You can use a timing light on each plug
wire to see whether the 'hiccup' coincides
with a certain cylinder spark drop...
three things, fuel, air, spark... one is
missing, could be fuel (sticky injector)
or spark (bad plug (it's happened!) or
cable, rotor, cap). Normally you'd feel
a bad position (crank,cam) sensor under
load as well as no load... you can check
your TPS (throttle position sensor) easily...
get yourself a Haynes, Chilton or (best!)
factory manual. Good luck.
__
Steve
..

they <ford dealer> tuned it up before I bought it, i was checking the
plugs & air filter & found they were new. i shot some carb cleaner
through the intake- did not help.

The plug wires look new, but how do you check them with an ohmmeter?

I'll pull the dist cap & look for obvious <crack> , i thought you could
not set the timing on these.

sounds like many possibilities.... after the timing light check i'll
head to advance & get a Haynes.

Thanks for the help!

Stephen Cowell wrote:

> "Robert Coffey" > wrote in message
> news:ghEme.9935$3u3.9116@trnddc07...
>
>>I just bought my first jeep! a 98 4.0 tj. when the engine warms up it
>>"hiccups" at idle or when running at low rpm not under load. 81k miles,
>>new air filter, spark plugs, plug wires look good. any suggestions?
>>
>
>
> Did it do this before the tune-up? We need to know whether
> something in the tune-up did this, or not.
>
> Might be time for an O2 sensor (or two...).
>
> Have you checked the plug wires with an
> ohmmeter? How's the cap and rotor?
> Vacuum advance working? Timing set?
> No vacuum leaks?
>
> You can use a timing light on each plug
> wire to see whether the 'hiccup' coincides
> with a certain cylinder spark drop...
> three things, fuel, air, spark... one is
> missing, could be fuel (sticky injector)
> or spark (bad plug (it's happened!) or
> cable, rotor, cap). Normally you'd feel
> a bad position (crank,cam) sensor under
> load as well as no load... you can check
> your TPS (throttle position sensor) easily...
> get yourself a Haynes, Chilton or (best!)
> factory manual. Good luck.
> __
> Steve
> .
>
>

"Robert Coffey" > wrote in message
news:XOHme.9705$qJ3.3993@trnddc05...
> they <ford dealer> tuned it up before I bought it, i was checking the
> plugs & air filter & found they were new. i shot some carb cleaner
> through the intake- did not help.

That's what I was thinking... you don't know that the
tuneup hurted, or helped. You didn't get to see the
plugs that came out, or set the gap on the ones that
went in.

> The plug wires look new, but how do you check them with an ohmmeter?

Your Haynes/etc. manual will tell you... basically, they're resistor
wires, made of carbon-impregnated rope covered in silicone rubber.
Ohm from one end to the other.. look for one way out. You'll get
a feel for how many ohms-per-foot by the time you ohm them all.
Let us know if you need lessons on how to use an ohmmeter...
it's pretty easy, and what with cheap DVMs around $30, it's
one tool that's *way* useful you can afford... many, many uses.

> I'll pull the dist cap & look for obvious <crack> , i thought you could
> not set the timing on these.

Yeah, you can still check it, though... and watch for advance, missed
sparks, etc.

> sounds like many possibilities.... after the timing light check i'll
> head to advance & get a Haynes.
>
> Thanks for the help!

You're quite welcome! Don't forget to look for big vacuum leaks...
__
Steve
..