'90 V3500 350ci TBI - idles too high & stumbles in drive/reverse

[Mungo]

Hi,
I'm looking for some help in the steps to diagnose an idle problem.
Started from cold the idle is high and after a short time gradually reduces, but ends up a bit high as it bangs when taking TH400 from park straight to reverse (the workaround is to go straight into drive from park as softer, and then quickly into reverse while revs are still dragged low).
The second issue is that when at normal operating temperature, the engine will stumble at idle, say while turning around in the yard or waiting in traffic (the stumble has to be caught by tapping the throttle pedal).
This is an ex-USAF crewcab truck with no emissions equipment fitted, other than the PCV and I have checked that for sealing by pulling the hose and sealing the vacuum port (also swapped over another PCV valve), same symptoms.
I'm not so proud of the fact that it's been like this for a year or so, the symptoms have not changed at all over time.
I've suspected an intake manifold leak and the IAC valve but don't have the diagnostic experience to isolate the cause.

Please lend your experience as I am now determined to sort this out.  If you can suggest some tests and checks I'll much appreciate this.

[VileZambonie]

Check for vacuum leaks. Listen around, spray the base of the TBI with some carb clean. See if it impacts the idle speed.

[Mungo]

Hi, thank you VZ, I'll give that a try.

[bd]

Regarding the second issue, TBI is very fuel pressure sensitive.  While you're in the initial diagnostic stage you should measure the fuel pressure using an appropriate gauge.  Blow through the fuel filter to make sure it is not restricted.

[Da67goatman]

Not trying to hijack the thread, but what fuel pressure tester do you use for TBI?  Seems there is a very specific adapter that doesn't come with any of the kits I've seen.

[Mungo]

Hi Rich,
Thank you, I’ll check out where the fuel filter(s) is/are and work out how to tap into the pressure; I guess an inline tee fitting at the TB end of the filter?

[bd]

Early GM TBI runs fairly low fuel pressure (~11 PSI).  Most fuel injection pressure test sets contain 100-PSI gauges.  A 100-PSI gauge doesn't provide sufficient resolution to accurately evaluate an 11-PSI fuel system.  You can rig up a TBI specific fuel pressure tester using a 15- or 30-PSI gauge, an inline adaptor (two styles imaged below) that substitutes for (or fits inline with) the factory fuel filter located inside the right frame rail next to the transmission, and a suitable hose with fittings to connect the pressure gauge to the adaptor.  Note that the interconnecting hose should be sufficiently long to monitor the fuel pressure from inside the cab while on a road check if necessary to diagnose intermittent symptoms. 

Now, a word of caution!  Due to the extreme potential for pressurized fuel leakage and fire, the test gauge and hose should never enter the cab!  Instead, secure the gauge under a windshield wiper arm in such a way as to be visible from inside the cab during any road test.

[VileZambonie]

I permanently install those adapters on my trucks so I don't have to remove the fuel filter to check fuel pressure. I also keep at least two in stock because removing and reinstalling fuel filters is a smelly mess. Definitely just buy a TBI gauge setup. I have several EFI kits and they cost a lot of money but here is a cheaper one with all the TBI adapters I have which will do more than what you need.

[Da67goatman]

I permanently install those adapters on my trucks so I don't have to remove the fuel filter to check fuel pressure. I also keep at least two in stock because removing and reinstalling fuel filters is a smelly mess. Definitely just buy a TBI gauge setup. I have several EFI kits and they cost a lot of money but here is a cheaper one with all the TBI adapters I have which will do more than what you need.

Do you have a part number for that kit or can you recommend a substitute?

[VileZambonie]

I do but would suggest you don't buy a MATCO one. The warranty isn't good, no better than Harbor Freight. Maybe get a harbor freight one or some place local to you.