The vehicle is a '92 Superduty w/460 auto. The main symptom is that of the classic vacuum leak (must keep rpm very high to run), and developed gradually over a period of 3 to 4 days of driving to the point where rpm had to be kept so high that one couldn't shift into gear. Eventually the truck could no longer even be started.
Been trouble shooting this one for about a week. Engine will now only run if fuel is directly squirted into manifold, and then it runs smoothly for several seconds. There is proper fuel pressure at the injectors, and a stethscope placed on an injector early on produced a weak but distinct clicking sound so I assumed the injectors were operating which made me focus on sensor/vacuum problems but I couldn't find any such problems. What I do now know is that both wires connected to each injector shows just shy of 12 volts, but when a noid lamp is connected, the flashing is intermittent and very very weak (have to cup noid lamp in had to see the flash). Evidently not enough of a signal to open the injectors. Stethscope now produces very very weak or no click. Problem seems to be continually worsening.
All sensors check fine, but they shouldn't matter if they are not in the starting circuit for the engine anyway.
Has anyone seen this symptom before, and perhaps have a suggest of where to look next? I hate to replace the PCM unless I'm certain, but I don't know what else to do here. I'm assuming since the sensors, which are activated by the PCM, have proper voltages, and since the pump/ignition seem fine, perhaps the PCM is at least mostly ok .
Does one of the pins from the PCM serve to ground all the injectors? Perhaps that is where I should look?
YEs, I did find some trouble codes, but since the codes had never been read in the truck's 15 years, I'm not sure how relevant they are (old codes). In anycase, there were three: Fuel pump (recently changed, and produces fine pressure at fuel rail), EGR control (shouldn't affect starting, and I removed and plugged it as a test anyway, thinking it may have been stuck in the open position...it wasn't) and the MAP sensor was coded to be providing improper readings. I checked the MAP sensor and it is within specifications...and besides, it shouldn't influence the engine start, right?
EGR and MAP can very much affect starting (especially the MAP). Codes are very important to diagnosis in these trucks, and are not stored forever, only so many drive cycles.
Go to fordfuelinjection.com to learn how to pull your codes and then post them here.
Jason
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-1990 F150 XLT Lariat LB 302 2wd AOD 3.55 open (former work truck, gone, but not forgotten)
-1993 F250 XLT xcab LB 351 2wd E4OD 4.10 open (parked, tranny issues)
-1989 Mazda B2200 SB 2.2L 2wd 5-speed (work truck)
Ok, here are the codes and what my Haynes says they are for:
556: Primary fuel pump circuit failure. But there is pressure (35 to 40 psi) at the fuel rail.
126: MAP/BARO sensor out of range. I checked sensor in accordance with specifications obtained from manual and, curiously, if I used the return line on the sensor for ground (digital meter reading frequency) I got readings within the normal range; however, if I used chasis ground instead, the indications was not only out or range, but the trend was opposite what it should have been. According to Haynes, either the return line or chasis ground could be used for the ground on the meter, but the results are very different. This is the test that make me suspect that grounding is the problem here. PErhaps I should try to start with the MAP disconnected. I honestly don't remember if I haven't already tried that, but I'll do it anyway.
327: EGR (EVP/PFE/DPFE) circuit below minimum voltage. As I said earlier, I completed removed the EGR from manifold (duct-taped opening) and observed no difference at all. I don't know what all the other abbreviations are for. Should I look there? I will.
536: Brake ON/Off (BOO) circuit failure/not activated during the KOER test. Well, unfortunately, I never got to the KOER test since the engine won't run.
542: Fuel PUmp circuit open; PCM to motor. Again the fuel rail has plenty of pressure. Not sure what the PCM to motor refers to? Any ideas?
The MAP sensor is the primary sensor the computer uses to determine fuel requirements on speed density systems (rpm vs. MAP, aka speed vs. air density). As you've seen, if it is not sensing correctly, it will cause starting problems. I hope the new sensor will get your truck back to normal. You may also want to check the computer ground (due to the discrepancy when measuring MAP signal) and the 12V supply from the EEC relay (due to the weak voltage at the injectors -- you should see full battery voltage on each injector wire with key on engine off and the injectors plugged in).
Unfortunately I'll be out of town the next 5 weeks, so when my brother in law changes the MAP sensor (it's his truck) I'm hoping all goes well.
REgarding the voltage at the injectors, it was very close to 12 volts: it was 11.93 volts for each wire. I have to admit the discrepency between the output of the MAP sensor using the sensor return wire (which gave a normal reading using a frequency meter on my DVM), and using chasis ground instead of the sensor return wire (which gave me nonsense) does bother me and does seem to point to a grounding problem with the computer. That said, it'll just have to wait until I return if the MAP sensor doesn't solve it.
Thanks yet again. Great forum. I'll keep watching for anything I can provide some help for.
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