Yeah I have one and am going to look tonight and see if it draws a heavy vacuum at tip-in.

 

Your 1992 truck has a vacuum servo cruise control setup, assuming you have factory cruise control. Most have a small vacuum reservoir behind the driver side headlight. Conveniently hidden away....

On my old 1992 F350 this small can was metal and rusted through.

 

I'll double check but no cruise for me or power windows or locks heated seats Lol, It's just an old fleet truck, I'm lucky it has AC although it is R12 and I'm going to have to convert it.

 

No cruise means there is no reservoir. One less thing to worry about.

 

Ok it's pulling 20 in Hg at idle.
As I slowly open the throttle it dies this happens just as the throttle lever leaves its seat.
It doesn't matter how slowly I do this it will even stall if I turn the idle screw to adjust the idle up it stalls.... When this happens the vacuum is still 18 in Hg.

When I plug the vacuum line for the brake booster the vacuum remains @ 20.

The vacuum reservoir output is 17 in Hg and bleeds off slowly after I turn the ignition off.

When I get more time I guess I'll pull the throttle body and inspect it from the inside out.

 

Still have not found the problem thought maybe it was the vacuum line for the map sensor so I replaced it but it went right back to stalling out.

 

check fuel pressure

 

Thanks see page one

42 psi @ idle 50 psi w/ regulator vacuum disconnected
Disconnect regulator and plug vacuum line.
Fuel pressure stays at 50 psi and engine still dies @ tip-in.

 

You have replaced a bunch of parts but have you verified the new ones are good? i.e. did the old TPS test bad. Is the new one within specification? Just because a part is new does not mean it is good.

You can have a vacuum leak, enough to cause driveability issues, but you will never see it with a vacuum gauge. On the other hand a vacuum leak will typically cause a high idle as well.

 

Thanks for the interest.
I have worked my way around the engine with a can of starting fluid spraying each vacuum line and have fixed a few small leaks and it idles steady about 700rpm.

When I resistance checked the TPS I had a spot that dropped in ohms and it was one of the few parts the PO had not changed and I thought. AhHa I got it, took my meter with me to pick up the new one and checked before I left the store. Installed it and had the exact same problem.

I feel it could be a vacuum circuit that is closed at idle that opens prematurely at tip-in. But which one ???
With these stupid complex emissions systems it is hard to pin it down.
And a Hanes manual isn't enough.

 

There is no tip-in vacuum circuit. Measure the TPS output using your meter set to DC volts. Slowly open the throttle while observing the meter. Look for dips, spikes, etc. Be aware if your meter is auto-ranging it can trigger a flash reading, I prefer setting the range myself.

 

Thanks, for the input.

Starts at about .84 VDC closed throttle
Stops at about 4.6 VDC at WOT

Voltage goes up evenly while opening the throttle.

Where could I measure the output of the ECM ?
As the throttle opens is it telling the injectors to open for a longer duration ? Could I track that?

If there are no vacuum leaks it must be a lack of fuel, so what would be telling the injectors not to open?

What is the equalivalent in this system to an accelerator pump in a carberator ?

 

As long as the output is linear with no dips or spikes, especially slightly off idle then you can eliminate that as a cause.

It's a combination of many sensor inputs to the PCM (TPS, ECT, ACT, MAP, PIP) that come into play for fuel/timing strategy. There is no accelerator pump per se.

You could watch the injector pulse width with an oscilloscope to see if or when the injector pulse is increased.

It may be worth the time and effort to baseline the other sensors, MAP/ECT/ACT at ambient temperature, ~100 then operating temperature to see if one is outside normal range, but not far enough to trigger a code. Beyond that you are looking at basics, fuel and air. Or even further down to basics such as compression.

There is also the possibility of a PCM going bad. The capacitors are know to leak out onto the circuit board. Another worthwhile check is to pull it out then inspect the inside.

EDIT: Looking back through this thread you mentioned before the throttle had a "sticking" point when you tried manually opening the blades. Did I dream that? Did you ever pull off the TB to inspect it? The throttle operation should be very smooth from closed to fully open.

 

I hope to pull he throttle body and inspect it.

It would be nice to put the engine on an oscilloscope but that would be way out of budget for me and it's getting crowded under my shade tree. Lol I gotta keep laughing I just need an old truck and this thing is kicking my a$$.

I'd have to borrow a compression tester to check compression to check my valves but I have doubts about finding bad rings or valves other than stalling when you just begin to open the throttle this engine runs STRONG and has no miss or flutter.