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-   -   1990 F150 4.9L High Idle Problem (

joemac1mn 02-02-2016 09:47 AM

1990 F150 4.9L High Idle Problem
This truck will not idle down and has poor gas mileage. I have confirmed the IAC is operating. I checked it with a scan tool. KOEO(slow and fast): No Codes. KOER(slow and fast): 13; Cannot Control RPM During Self Test Low RPM Check. (I could tell the computer was trying to slow the idle, but it wouldn't slow.)

I checked the live data stream with these results.

ECT(V) 0.6; ECT(F) 194; Engine speed 1328, EVP(V) 0.3; EVR(%) 0; FuelPW1(ms) 6.4; IAC(%) 20; IAT(V) 0.7; IAT(F) 180; Loop Status Closed; MAP(hg) 10.0; 02SEN 1(V) 0.27; Spark 14; ST FTRM1(%) 8; TP Mode C/T; TPS(V) 1.0; V PWR(V) 14.1; V REF(V) 5.0

From what I have checked, the IAC at 20% is normal. Any suggestions, on what to do next, would be appreciated.

subford 02-02-2016 10:56 AM

Smoke test for vacuum leak.

joemac1mn 02-02-2016 12:12 PM

I have sprayed the vacuum lines, Intake Manifold area, and everywhere else I can think of, looking for signs of a leak, but no luck. I know that method doesn't work for detecting all leaks, but made the attempt, as I don't have the equipment for a smoke test. I'll see if I can get a tester.

joemac1mn 02-04-2016 02:10 PM

I retested, with scan tool attached and set to ST FTRM1(%). It had showed me 8% when I first scanned, but the engine was cool this time, but showed a Closed Loop, and the percentage was from 1 to zero.

I then began spraying the intake gasket area, where it attaches to the engine and the readings immediately dropped to a low of -21, then slowly recovered when I stopped spraying. I had expected to see a rise in percentage if the intake manifold gaskets were bad. I didn't detect a change in idle, but tried this on another vehicle I own and fixed an intake manifold gasket issue.

My question is, at this time, does anyone know for sure what the negative numbers mean? I'm guessing that it might be the rich air is entering the exhaust manifold, but I don't like guessing.


rla2005 02-04-2016 03:21 PM

In my opinion the negative Short Term FT value is pulling fuel out because the Long Term FT has not had enough time to adjust. That's been my experience with exhaust and vacuums leaks since I started using a Data Logger on my new to me 2005 Navigator.

Others may have a more experienced viewpoint.

danr1 02-04-2016 04:15 PM

The smoke test is the best way to find vac leaks if suspect there is any.

Simple enough to do and building a home spun smoker out of something very likely already have on hand is easy enough too.

If you don't own a compressor and pressure regulator that would be a snag though, you'll want to limit it to 1 or 2 psi behind the smoker.

I used a garden hose and slow but steady stream of water for years to check for vac leaks before putting together a smoker.

Works great, let engine and exhaust system get up to full operating temperature first. Take the hose and working low to high front to back allow small but steady stream of water to flow where suspect a vac leak, like along the valley gasket and head junction on each side of a v8.

Have someone watching tail pipe at the same time, if water finds a vac leak steam will immediately appear in the exhaust stream. Big leak lots of steam smaller leak not so much but there will be noticeable steam if you hit a leak with the water.

I've never burnt anything out never stalled one out and for example I've flooded injectors during the test engine running, using water it just plain works for finding gasket related vac leaks.

The vacuum lines are easily tested with a handy vac so no need to literally flood everything trying to check vacuum hoses with the garden hose.

joemac1mn 03-20-2016 11:50 PM

Update on fix.
I was finally able to get a smoke machine to test the system. It was a bad intake manifold gasket and small leaks at the Egr and throttle body gasket. I put in new gaskets and a new Egr valve. Idles as it should and I'm still checking the gas mileage to see how much that helped.

Thanks for the advice. Nice to have it fixed.

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