High idle, '88 5.7

[NMNorsse]

In park at full temp in a heated garage my '88 k1500 5.7 idles pretty high. I'd guess 1,200 but I don't have a tach. Put it into drive and it idles around 700-ish, which seems fine. It is the fast idle that I'm not so sure about.

To deal with a variety of issues, including age, in the last year I've replaced PCV, CTS, TPS, O2, EGR, IAC, AIR pump, Intake Gasket, TBI base gasket & fuel line o-rings, pressure regulator diaphragm & spring, fuel pump assembly, fuel filter, plugs, wires, cap, rotor, water pump and hoses. MAP tested good and checked for vacuum leaks with propane and carb cleaner without finding anything. Unplugging the brake booster does make idle go up. I also pulled the ECM fuse for 5 minutes, put it back in, turned everything off, set the brake, put it in drive and let it idle for 20 minutes to relearn. It idles smoothly at all times, no stumbles or surges.

Is that fast idle in park at temp the way it ought to be, or am I missing something? It seems too high and I'm out of ideas.

 

[someotherguy]

Looks like word-for-word your previous post I replied to except I don't recall the "at full temp" criteria.

So I'd say first thing is let's eliminate the guessing at RPM's. You can attach a diagostic style tach easily at the extra white wire terminal near your coil. That's a tach signal output. Usually the connector is taped to the flex loom right at the coil, so you may need to unwrap that small amount of tape.

You must be registered for see images attach

Richard

 

[NMNorsse]

Looks like word-for-word your previous post I replied to except I don't recall the "at full temp" criteria.

So I'd say first thing is let's eliminate the guessing at RPM's. You can attach a diagostic style tach easily at the extra white wire terminal near your coil. That's a tach signal output. Usually the connector is taped to the flex loom right at the coil, so you may need to unwrap that small amount of tape.

You must be registered for see images attach

Richard

Thank you for the response.

I'll get a diagnostic tach and fuel pressure gauge to check those things. I think one of the parts houses has the fuel pressure gauge in its loaner fleet.

I didn't know that is what that white wire is for, I just had my hands on it when I was replacing the intake gasket. Thanks for telling me!

I downloaded the IAC calibration stuff from a different post on '88 350 TBI rough idle when warm' too.

I did post a comment in another thread on rough idle. Then I figured I'd make it into a new post in case someone else has the same question and searches for it, so I cut and paste it here. Hope I didn't violate some site etiquette doing that.

 

[someotherguy]

The fuel pressure gauge in the pic is just because that was the original purpose for the pic, to show another possible location for the gauge tee. Sorry, wasn't trying to imply fuel pressure needed to be checked in your particular scenario! It was just the pic I had handy that also showed that connector for the tach.

Richard

 

[Schurkey]

In park at full temp in a heated garage my '88 k1500 5.7 idles pretty high. I'd guess 1,200 but I don't have a tach. Put it into drive and it idles around 700-ish, which seems fine. It is the fast idle that I'm not so sure about.

Yeah, that's not right, assuming you're accurate about the idle speed.

To deal with a variety of issues, including age, in the last year I've replaced PCV, CTS, TPS, O2, EGR, IAC, AIR pump, Intake Gasket, TBI base gasket & fuel line o-rings, pressure regulator diaphragm & spring, fuel pump assembly, fuel filter, plugs, wires, cap, rotor, water pump and hoses.

Were these parts replaced because they tested bad, or were they thrown on with no diagnosis?

What is the ignition timing at idle with the "set timing" connector disconnected?

Unplugging the brake booster does make idle go up.

The brake booster hose was left open to suck air? Yes, the idle speed increases.

Is that fast idle in park at temp the way it ought to be, or am I missing something? It seems too high and I'm out of ideas.

Connect a scan tool, verify ALL the sensors and computer outputs including the IAC.

Did you ever remove the tamper-proof cover over the throttle body minimum idle speed screw?

I did post a comment in another thread on rough idle. Then I figured I'd make it into a new post in case someone else has the same question and searches for it, so I cut and paste it here. Hope I didn't violate some site etiquette doing that.

Looks like word-for-word your previous post...

Ideally, you wouldn't post the same question more than once.

 

[CumminsFever]

I once had an s10 of the same era that did the same thing. It turns out there was 2 coolant temp sensors. 1 for the idiot light in the dash, 1 for engine management. The engine management sensor was bad, it showed cold all the time.
I don't know if these trucks have 2 coolant temp sensors. If so, maybe you replaced the wrong one?
Your symptoms would make sense if the computer saw a cold engine all the time.

 

[NMNorsse]

Thank you all for your input!

Timing was set with connector unplugged.

The vac line from the booster was pulled to see if the idle increased. If not, it was already leaking.

Most parts were replaced for failure-EGR, water pump, air pump, intake manifold gaskets, valve seals, Master Cylinder.

Others because I was in the area like thermostat, hoses, CTS, TBI base gasket.

I replaced the fuel pump assembly and pressure regulator diaphragm/spring because it cranks a little before it starts and I thought it was the check valve or a leaky diaphragm. Nope.

Others were regular maintenance, like the PCV, cap, rotor, plugs, fuel filter, etc...

Others tested okay but were 35 years old like the IAC, TPS & O2 so I decided to replace them preventatively and hoped they'd fix the high idle.

I have not removed the plug over the idle adjustment screw in the TBI body. I don't think that's the cause or the solution.

There are 2 temp sensors, I replaced the CTS in the intake not the one in the head that runs the gauge.

Tomorrow I'll check a different way for a vacuum leak in the brake booster: Blocking the hose to see if the idle drops. Brakes work okay but it's running like it's got a vacuum leak and that's the only spot I haven't fully tested.

 

[Scooterwrench]

May want to check that TB base gasket and make sure they gave you the right one. Spray carb cleaner around the base of the TB and see if the idle changes.

 

[NMNorsse]

I was wrong.

I got the truck last year after it had sat and was rarely driven for most if its life. Under 60k original miles and many of those possibly from being flat towed behind an RV by the previous owners since the speedo/odo are mechanical. One of the things that was broken was the EGR. After I replaced that and cleared up the other vacuum leaks the idle jumped a lot. Especially after I put in the new Idle Air Control valve (IAC).

It wasn't the brake booster.

I didn't think to look for the obvious until Schurkey brought it up: someone had taken the tamper plug off the idle adjustment screw. The black T-15 screw head has some scratches on it. I think the previous owner had a low idle and just dialed up the screw to fix it rather than finding the cause (probably the IAC). I've dialed it back down and going to see how it goes. Tonight I'll put a tach on it and make sure its 750 in park and 500 in gear when at temp. Maybe I'll ask a friend put his smoker on it to make sure I'm not missing a leak.

Thank you Schurkey!

 

[NMNorsse]

May want to check that TB base gasket and make sure they gave you the right one. Spray carb cleaner around the base of the TB and see if the idle changes.

Thanks for the suggestions.

I matched the new gasket to the old and the TBI body before I bolted it up. I tried carb cleaner around the base and it didn't make a difference.