I did also monitor the change in Voltage throughout the throttle's range of motion. It was smooth as you described it should be. Sorry I forgot to mention that.


Earlier today I swapped out the O2 sensor and it made no difference. I've got the TPS sensor and am going for that next. Following that, I'm going to follow the official Ford method for adjusting the throttle stop screw, as written up here.


I also have the engine coolant temperature (ECT) sensor, and will throw that in at some point.


Thanks again for all your ideas. I'll report back.


Dan

 

I got busy and the truck sat in my driveway for several months. It still has the same problem.

I replaced the TPS and ECT last year after my last post. No change.

Also last year after my last post, I went through the idle set screw procedure. The signal indicates that the idle is too high. When I turn the screw to lower it, the truck will die before the signal indicates the speed is correct.

Earlier this week I used a straw in the number one cylinder to find TDC, made a mark on the harmonic balancer, and adjusted my timing to 10 degrees BTDC. This made no difference.

After I adjusted the timing, I ran through the idle air screw adjustment procedure again. Same result. Reports as too high, but dies before it goes low enough to read OK.


I feel like I'm running out of things to try. Any recommendations will be greatly appreciated. It still has the issue where there's a lag between opening the throttle and rpms going up. It's significant - a half-second or so. Any ideas about that issue, which is hopefully the same problem as the 411 code and the dying at idle?

 

Did you ever replace the IAC valve?

 

Yes, I did replace the IAC with a new part last November.

 

I know another thing that was mentioned here was a worn throttle body, and that's something I can check/replace. If that was worn how would the truck behave?

 

It seems like I'm talking to an empty room here, but I'll keep posting in the hopes that this will help someone experiencing a similar issue.

I've read more about this, and it seems like a worn/leaky throttle body would explain my symptoms. From what I have read, both hesitation (the lag between throttle open and rpm increase) and dying at idle will occur if the system is running lean. When the throttle is at the closed position, the computer reads the throttle position sensor and meters in the appropriate amount of fuel for airflow through a closed throttle body. If air is leaking in, from around the plates in the throttle body or anywhere else, there will be too much air for the amount of fuel the computer is feeding the engine. This is a lean condition.

I've long since tested all my vacuum lines. I systematically measured the vacuum with a hand pump/gauge at each part of the system (as described earlier in this thread), and I replaced all the plastic lines with rubber. I've replaced the gaskets on the upper intake manifold and throttle body multiple times in the process of replacing the injectors and fuel pressure regulator, and I used gasket sealer in addition to the gaskets. I've gone around the entire intake manifold with starting fluid trying to detect leaks. I can put my hand over the throttle body intake and the engine will die quickly. All this leads me to believe that if there is a leak, it has to be in the throttle body itself. I've got a NOS throttle body on order, and it should arrive later this week. I'll report back on what I find.

 

Put on the new throttle body. Truck still dies. I was able to start the diagnostics right after I started the truck and it ran through the test successfully. It seems to keep itself running just fine while in test mode. When I did this, the truck threw the all-clear code. When I start it and run it outside of test mode, it dies fairly quickly as it did before, loping up and down until the down gets low enough to stall out.

Right now, my best wild guess is that the fuel injectors I put in at the start of working on this truck are not right. They're not the OEMs, and they didn't look identical to the ones that came out. I was thorough about making sure they were for this truck, but they are different from the originals. If the truck is acting lean and there isn't too much air getting in, it could be that there's not enough gas getting in. I can't think of another thing this could be.

If anyone has a suggestion it would be appreciated.

 

Did you ever block the PCV valve off? Either by a sheet of paper or some other way so its not pulling in vacuum?

Got a picture of the injectors you installed?

What is your fuel psi? Running and with vacuum line removed?

Another idea and I have read about this in the past. Cap off the lines running to the TAD/TAB solenoids and the valves that run behind the intake. Be sure that the Air Bypass/Air Diverter valve has Zero vacuum.

See if that improves the problem. I have read where the TAD/TAB solenoids can cause an exact problem with a rolling/loping idle. This is in the 1992 Ford Powertrain/Emission Book.

I'm hoping we can get you running again smoothly!

 

Hey, Timbersteel,

Thanks very much for the ideas. Some of these were things I've tried before, but I ran down the list regardless, just for the sake of argument.

I took it out of the valve cover, covered the inlet hole in the bottom with duct tape, and put it back in place. This made no difference.

I don't, but here they are: http://smile.amazon.com/gp/product/B...rch_detailpage
I don't remember how exactly they looked different from the factory parts. It's been too long to recall.

Put on the gauge and measured this again. I have already replaced a (bad) fuel pressure regulator, and I had tested it at that time. Today I measured 41 psi with vacuum, 36 psi with no vacuum. Later, when I had the truck kept running at high rpms, I measured again, and it read 43 with vacuum, 45 without.

I removed and plugged the vacuum line that supplies vacuum to the TAD, TAB, and EGR vacuum solenoids. On this truck this was the output line of the 'tomato can' vacuum reservoir. This made no difference, other than when I ran diagnostics it threw codes for the EGR and thermactor systems. When I did this I hooked up my vacuum gauge to the air divertor valve and it had no vacuum.

Thanks again for replying. Given all this, what are your thoughts now? Do you think I'm on the right track thinking it's the injectors?

Have a good one,


Dan

 

Your fuel psi is To high.

Running psi, with vacuum hose attached should be around 30-35.

With vacuum hose disconnected/cap, it should raise to around 39-40.

That loping idle is sure sounding like to high a fuel psi. It can't compensate for the excess amount of fuel being dumped in.

To test the theory of to high a fuel psi, remove a vacuum line from say the brake booster. You can use your thumb to adjust how much a vacuum leak you want to have. Now see if your idle straightens out and are not left with a just a higher idling engine. I mean it will be around 1500-1800 rpms, but the loping should have quit.

 

Put on the gauge and measured this again. I have already replaced a (bad) fuel pressure regulator, and I had tested it at that time. Today I measured 41 psi with vacuum, 36 psi with no vacuum. Later, when I had the truck kept running at high rpms, I measured again, and it read 43 with vacuum, 45 without.


The part I'm confused on is if your reading of 41psi with vacuum, 36 psi with no vacuum, is correct. It should be the opposite.

Usually, once vacuum is at and below 5psi, that's when the amount of fuel psi begins increasing.

 

4.9L engines run higher pressures than the V8s. Pressure is low to me. With no vacuum the pressure should be in the 55 - 60 PSI range.

 

I'll try pulling off a vacuum line and trying to keep it running that way. From what I remember from plugging and unplugging lines, it seems to want to die when a line is opened up.

Am I right in thinking that the hesitation it has indicates it's running lean? When I open the throttle, there's a roughly half-second delay before rpms increase. If that's so, then that would point to there being not enough gas, right? If the fuel pressure I read is low, not high, would the readings I saw make much difference? I seem to remember the readings being within spec after I replaced the regulator.

Let me know what you think.

 

Aww heck!! I thought this was a V8 engine...

Then, as RLA stated, fuel psi is too low.

 

So it seems that my fuel pressure readings are on the edge of the acceptable range. Do you think it's likely that this could be the reason it's acting this way? The hesitation and the loping/dying?