94 with 408 engine running bad

Jedeka · Jan 28, 2016

This truck has been running fine until a couple days ago, when I used it to plow some snow. What started out as a couple hiccups, has now turned into a hesitation, dying and rich running conditions. Fuel pressure is a steady 45 psi and will maintain the pressure after shutting off the truck. It seems to get worse after the engine starts getting warmed up. No CEL, spark is good as well as cap and rotor. I had my scanner on it and thought MAF sensor was running above the specs. Replacing the MAF sensor did not help. I removed O2 sensor without change. I swapped out IAC with a used one. Now engine will not idle without foot on throttle pedal. This leads me to thinking it is an IAC problem. The other day when it died, I had to try 10 times before engine would stay running. Sometimes when it died, it would start right back up. If it started to die and I gave it some gas, it would hesitate like a bad accelerator pump. Spraying a little either into the name pipe would not cause the RPM to increase. Pulling off the brake booster hose would cause the RPM to increase a little and seemed to run better. At one time I was thinking crank sensor, but now I'm unsure where to look. Thanks.

MagSport · Jan 29, 2016

Replace the iac with a new unit

Jedeka · Jan 29, 2016

IAC motor did not help.

MagSport · Jan 29, 2016

Then try the tps. If a crank sensor or cam sensor is bad, it wouldn't run at all. Btw the fuel pressure is kinda low. It should be knocking on 50psi, factory spec is 49. Also did you reset the computer when you changed the iac?
Info on the MAF sensor. These motors use MAP sensors and very rarely do they cause any issues. This being a 408, did you adjust the distributor any? And if so did you find someone with a drb3 tool to get it back in fuel sync?

Jedeka · Jan 29, 2016

TPS did not help. My factory service manual says fuel pressure range is 35-45 at idle. At 45, I was thinking I'm ok. How do I reset the computer after the new IAC? Thanks for correcting me on the MAF vs MAP. I knew better. I have not done anything to the distributor or fuel sync. I have a Perfect Power connected the the PCM wiring but have not been able to get truck on a dyno to tune from the base program. This truck has been running fine for the last 6 years with the engine. I'm sure some dyno tuning will benefit overall performance.

MagSport · Jan 29, 2016

Jedeka said:
TPS did not help. My factory service manual says fuel pressure range is 35-45 at idle. At 45, I was thinking I'm ok. How do I reset the computer after the new IAC? Thanks for correcting me on the MAF vs MAP. I knew better. I have not done anything to the distributor or fuel sync. I have a Perfect Power connected the the PCM wiring but have not been able to get truck on a dyno to tune from the base program. This truck has been running fine for the last 6 years with the engine. I'm sure some dyno tuning will benefit overall performance.

Alright if it's been in there awhile fuel sync isn't the issue. To reset the computer, disconnect the battery and hold the terminals together for 1-2 mins to discharge all of the capacitors in the system. Then plug it back up. The computer will then relearn all of the sensors and their values.

Jedeka · Jan 29, 2016

I reset computer. Nothing really changed. Seems like it will stay idling. If you open throttle body to 10%, the engine hesitates. If you go slow, it will rev up without a miss. Engine does seem a little lazy. I haven't driven it since putting on IAC and TPS.

Mohpar · Jan 30, 2016

What was the bit on removing the 02 sensor without change? When I had first swapped my engine in I didn't hook up 02 sensors, and they didn't throw any codes. Was running poor, dying at stoplights if I didn't gas it. Got new ones setup and the problem went away within a drive or two.

I'm thinking that's your problem

Jedeka · Jan 30, 2016

I was trying to eliminate o2 sensor and plugged converter by removing the sensor. According to scan tool, the o2 sensor is operating within range.

Mohpar · Jan 30, 2016

Jedeka said:
I was trying to eliminate o2 sensor and plugged converter by removing the sensor. According to scan tool, the o2 sensor is operating within range.

Hm. Any chance you can get a light and peek in the very back of your engine where the tranny bolts up and have a look at the crank sensor? Looking for wire damage.

Odd that it runs worse after getting warmed up. Still thinking on that one

Jedeka · Jan 30, 2016

I'm leaning towards bad crank sensor or o2 sensor.

MagSport · Jan 30, 2016

Jedeka said:
I'm leaning towards bad crank sensor or o2 sensor.

O2 sensor can cause these symptoms. Especially if it's worse after it warms up.

Jedeka · Jan 30, 2016

O2 sensor didn't help so I changed crank sensor. Still no difference. I'm pulling all of the plugs to check them. If I tap throttle, the engine will go to 1000 rpm, then back down to idle. After doing that 4 or 5 times, the engine dies, but starts back up. Almost acts like it floods the engine out.

Jedeka · Jan 30, 2016

Found the problem. Ended up bypassing the Perfect Power module with the piece included with the module. Problem is gone, but exhaust stinks. Hopefully, I caught problem before doing any damage to convertor. Thanks to all that helped.

94 with 408 engine running bad

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