Hey guys. We've got a '94 Econoline Club-Wagon with a 4.9 (I know it's a van, but this section is more active the van section) in the shop that is giving us quite a headache.

Cold it runs fine, but as soon as it gets some heat in it and kicks down to the normal idle, it starts to run very bad. It will start to missfire and begin to stall out, then the computer will bring it back in line for half a second or so then it will start to missfire, and the process just keeps repeating over and over. If you leave it long it enough it will eventually stall out. When under acceleration you'll hear popping noises in the exhaust, it missfires, has low power and pings/knocks.

Fuel filter has already been replaced, owner had replaced the TFI module already. Vacuum at idle checks out, fuel pressure is ~60 PSI at idle, ~ 65 PSI @ WOT.

We've changed the cap and rotor (they needed to be replaced anyways), still runs the same. Unplugging the SPOUT connector makes no difference (well runs a bit worse with it unplugged actually). We're pretty sure the timing is good. It appears the outer ring on the damper rotated on the rubber, so we had to use the wire-down-cylinder 1-trick, find TDC and make a new mark. The engine behavior does not change regardless if the distributor is at full advance or full retard (it of course runs differently, but it still missfires, tries to recover, etc.)

We hooked up an OTC4000 scan tool to it, which is capable of doing live data stream (!). ECT, IAT and EGR status values were all in range. MAP was reading 106Hz at idle, which from what I gather is acceptable. TPS was at about 1 volt.

We headed to the internet, and actually found someone having a similar issue here. They said that by unplugging the MAP, their 4.9 started to run fine. They never had a follow up saying what the actual issue was.

We tried the same, as soon as we unplugged the MAP, the engine came to a steady smooth idle. Of course during acceleration it has severe hesitation because it has no MAP input, but no more knocking, popping, etc..

I pulled the MAP sensor out of my truck, it did run fine several months ago before the fuel pump died, so I assume the MAP is still good. We put the MAP from my truck into the van, and it started behaving the exact same way it was with its own MAP. Unplug the map = runs fine, plug it back in = runs terrible. If you take the vacuum hose off the MAP with it still plugged in, it instantly dies.

We also tried unplugging the O2 sensor, it made no difference.

We're at a loss here. The only thing we can come up with is that when we unplug the MAP, the computer reverts to some kind of default fueling table. We have no idea what is going on when the MAP is plugged in, because the OCT reports that the MAP, IAT and ECT are all in proper range.

Any ideas?

 

When the MAP is unplugged, it will go into default. Similer to open loop. It only looks at certian critical data to run. When the engine is running poorly, what's your fuel trim say, how bout O2 sensor data? Is it in open loop? MAP is the main calculator for speed density EFI. Have you tee'd into the vacuum source for the map to determine it has a quality sense of vacuum? You may also try disconnecting the MAP vacuum line and plugging it off. Install you're mighty vac and bring it to 10". Start the engine and bring your pump to 20". As long as the engine is in closed loop, you should see the fuel trim change. See if you can't get the engine to smooth out by playing with vacuum levels. 106 HZ is right on for 18" of vacuum.

 

Check the PIP signal from the hall effect sensor? Or did you happen to have timed off the wrong mark? Use the degree marks on the balancer, not the timing cover. Then you go off the pin on the passenger side. Or theres a separate mark for the driverside but I found that one is a pain. My '96 was really picky on having exact timing.

 

Forgot to check fuel trim, will look into it tomorrow. I'll see if it can display O2 data, but as I said, we tried unplugging it and it made no change.

We actually disconnected the MAP, and used its vacuum line for the test. We were getting about 20" at idle. I'll definitely see about hooking the hand vacuum pump up to the MAP, and playing with the various vacuum levels.

We thought about hooking the scope upto the PIP wire, but the engine runs fine with the MAP disconnected, and we do not have erratic spark timing. We're pretty sure that ignition system is not at fault.

The timing marks on the damper are worthless to us right now, the outer metal ring on the damper physically rotated on the rubber bushing. The timing marks are almost 180* out.

 

The reason I mentioned to look at O2 data was to give you an indication of weather the engine is running rich or lean. This may help you into what direction to go. Remember that 400 MV is bias, anything lower than that is lean, above that its rich. O2 sensor works on a 0-1 volt scale. Just because you unplugged the MAP, doesn't nessasarly isolate a failed MAP sensor. You may be able to duplicate the same symptoms by unplugging the ECT,IAT or TPS. Will depend on ECU software and what puts the engine into default.

 

I don't think the problem is the MAP sensor, because we tried another MAP sensor from a truck that was known to run fine and the van still ran terrible.

I was saying I don't think it is an issue with the spark, because when you do unplug the MAP and throw the computer into its default mode, the engine runs fine.

 

Good luck IAN, please let us know what you find or if I may be of anymore help.

 

rebuild or replace distributor. I know you've already eliminated it. That's why I'm bring it up again.

 

Fuel pressure is supposed to be 39psi above whatever is in the intake manifold. As most pressure gauges read relative to atmospheric, this would be about 30psi at idle and about 38 at WOT. 60 is way too high.

Did you check codes and run the KEOE and KOER tests?

 

My 1990 Service manual says 45-60 psi for the 4.9L EFI engine running, 50-60 KOEO

So yeah, 65 seems High.

 

Has anybody pulled the codes yet? Why do so many people ignore this step?

60psi fuel pressure is about normal for the 4.9 too.

 

I guess you missed the spot where I said we hooked up an OCT4000 scan tool to it...

KOEO: TCC Clutch and EGR Flow Low
KOER: SPOUT

SPOUT was because we disconnected the SPOUT connector. Not entirely sure about the EGR flow low code. EGR does not appear to be stuck however, since like I said, it runs fine if you disconnect the MAP.

 

OTC4000 did not have the ability to datastream fuel trim or O2 sensor data. Not sure if it's a limitation of the EEC-IV, or the tool it self. We hooked up the hand vacuum pump to the two map sensors we had on hand; at the same vacuum level both sensors reported the exact same frequency, so I'm guessing they are both okay.

If we manually pumped the vacuum up to about 18-20", the engine smoothed out a bit, but no where near acceptable smoothness. Plugging the truck's own vacuum supply line back into the MAP would make it run worse.

IAT was reporting 147*F, which seemed high to us, but the infrared thermometer said that was actually spot on. Probably because the exhaust manifold is right there next to it. Unplugging the sensor yielded no change in operation. TPS is at ~.95 volts at idle, seems to respond normally when you give it gas. EGR Actuator position was reading about .45 volts, which from what I read means the EGR is closed. ECT holds steady around 190* once the engine warms up. Sensor reference voltage is 4.98 Volts, Charging voltage is 14.4

We're out of ideas here. The only thing we have is that unplugging the MAP makes it run fine - except for the extreme hesitation while accelerating.

Any ideas at all would be greatly appreciated.

 

Anything guys?

 

ive been having the same issue as you, actualy i think you may have read my post about unplugging the map sensor and it running better. anyway i never followed up because i never figured the truck out and i parked it in my back yard until two days ago. however i now need to pinpoint the issue because it is stick and i have absolutely no low end power taking off from a stop, i replaced the computer with no difference at all so on that note i guess we can both help each other