The slight idle flux is normal operation on on of these SD trucks. My '94 Lightning does the same thing. As long as the idle is correct, when the truck is in gear, I wouldn't worry about it.

I would fix the exhaust leak, replace the fuel filter, and drive it.

 

http://www.oreillyauto.com/site/c/de...nt+temp+sensor

The above part is the type Temp Sensor you replaced, correct? The little burp/blip in idle, is normal for our SD trucks. My '94 Lightning does the same thing, once and awhile. I would fix the exhaust leak, replace the fuel filter, and drive the truck. If idle speed is correct when in gear, don't worry about it.


Sorry for the somewhat duplicate post, my tablet is just warming up, LOL.

James

 

Yes, that is the coolant temp sensor that was replaced.

So far I've done: TPS, IAC, AIC, MAP, ECT, O2, PCV, front brakes, shocks, plugs, wires, air filter, crankcase filter, oil + filter, flushed and recharged the coolant system...and cleaned a big field mouse nest out from under the intake manifold Seems I'm slowly restoring it one sensor at a time, but I don't mind, since until now I don't ever remember doing anything but brakes, and oil....embarassing to admidt, but she's just kept running, and after 21 years she looks like she just rolled off the line

I would just tolerate the fast idle in park, as suggested, but it will eventually kill the transmission...which goes CLUNK when it shifts into gear from the fast idle.

Over the next two weeks, prior to a big trip I have planned with it, I will fix the exhaust leak, do the rear brakes, service the manual locking hubs, and repack all the wheel bearings, have all of the seals in the transmission replaced, and while the transmission is out, replace the rear main seal.

I've very hopeful that all who pointed to the O2 sensor were right on track, and fixing the exhaust leak will be the idle solution.

Blue07STX: what does 'SD' mean?

As always, thanks for the input.

 

Speed Density (non Mass Air). I don't know if the exhaust leak being repaired will fix a high idle issue. Is the clunk a u-joint going bad? No need to pull the trans unless it has issues. A new trans filter, draining the torque converter and replacing the valve body with a Punisher street valve body will extend the life of the transmission.

I would disconnect the battery, remove the parking brake assembly on the drivers kick panel (three nuts) and check for an aftermarket chip plugged in to the end of the ECM. But, you are the original owner.

Not many items left to check, for a high idle condition. Have you replaced the fuel filter within the last two years or 20,000 mile? If not, it could be clogged. I would also check timing, just to ensure it is correct and the fuel pressure at the rail, engine off and engine at idle. You will need a fuel pressure gauge and correct fitting for your fuel rail, maybe $40 or you could borrow one from a parts store.

Glad to here you are getting it fixed up after 21 years. Time flies by as we get older, never seems like enough hours in the day. Oh my, now I sound old.

James

 

Well, the U-joint question is a little beyond me, but I tend to say it isn't that since when it shifts near the normal idle speed, it shifts smooth as silk. This whole thread started when, for a long time, I thought I had a transmission problem, but then (ever so slowly) I realized that it was my high idle that was causing the transmission 'thunk' when shifting. And soooo the saga began! Perhaps I shouldn't be trying to do my own maintenance I may kill my truck, but I am having some fun.

The fuel filter is in-hand and on the list to get done.

Original owner, so no aftermarket chip, and I've been shopping for a timing light to check the timing, which I'll get to in short order.

Now the BAD news:
I was tackling the exhaust leak today and, surprise...surprise...surprise...(a la Gomer Pile)...I tracked an oil leak to what appears to be the head gasket slightly forward of and beneath plug #1 on the forward passenger side. I think this is the exhaust leak sound I've been hearing.

What I don't understand is how I still have good compression in all of the cylinders.

I think I'll have to find a shop for an estimate.

Any opinions on replacing the gasket vs. a rebuilt engine vs. rebuilding this engine would be appreaciated.

Thanks alot for the support.

 

Well guys...in view of my blown head gasket diagnosis, I think we can end this thread.

After pricing various options, I've decided to pull the motor and rebuild it.

THANKS for all of the assistance on the idle problem, and I look forward to any help you may offer as I post questions on the rebuild.

Thanks again.

 

A final note...

I have pulled the engine to rebuild it, and in the process discovered a couple of vacuum lines that appeared intact, but upon disassembly disintegrated in my hands.

Hats off to those touting a vacuum leak as the likely cause of the fast idle.

Thanks.