87 TBI misses when hot

[johnnyr1965]

Ok ready to pull out my hair..need advise.87 GMC 4x4,350 TBI,mild cam,headers 4-speed.Runs great with plenty of power until after about 30-45 min and engine warms up it starts missing like its running out of fuel.Replaced fuel pump,injectors,plugs wires etc.Cleaned fuel tank,replaced fuel filter.When i had my dual pipes put on they ran the pipes about 3 inches from my fuel filter mounted under frame.My mechnic said the fuel filter is getting hot and vapor locking it.Just wondering of anyone has heard of this happening before i rerun my fuel lines....

[zieg85]

It is possible especially if it ran good before the dual exhaust.  Is it possible to make a heat shield for the filter or re-route the exhaust pipe some?

[bd]

The fuel filter is pressurized to ~11 PSI.  Although warm fuel isn't optimum, it's probably not the cause of your complaint.  Connect a fuel pressure gauge to the fuel system and drive normally until you feel the symptoms.  A fuel issue will show up as low pressure.  You should also check for trouble codes.  Keep us apprised of what you find.

[rich weyand]

Change the water temp sensor that's connected to the ECM.  If the computer doesn't KNOW the engine is hot, it could be giving it the wrong mixture (though I would expect cold mixture to be richer, not leaner; mileage would go to heck, but it should run OK).

[johnnyr1965]

Never drove it before duals were installed.I think i am going to get some of that heat wrap and cover filter and try it.Is not showing any fault codes other than o2 sensor which was removed when i installed headers and pipes.

[zieg85]

You need the O2 sensor to run properly

[johnnyr1965]

Never thought about that really. Guess i can drill a hole in pipe and install O2 sensor.

[rich weyand]

Uh, yeah.  The O2 sensor is how the computer decides how much gas to give the engine.  Didn't even consider that the O2 sensor wasn't there.  What you need is called a bung.  I had one brazed into my exhaust pipe behind the header flanges (for my air/fuel ratio meter) for $30.  Probably an 18mm, which is pretty much the standard, though you should check to be sure: http://www.summitracing.com/parts/aaf-all34153?seid=srese1&gclid=CLejn9qi87cCFc9DMgodrWsA2g

[bd]

Never drove it before duals were installed.I think i am going to get some of that heat wrap and cover filter and try it.Is not showing any fault codes other than o2 sensor which was removed when i installed headers and pipes.

With no O₂ sensor the ECM is likely defaulting to "Limp Mode," which provides just enough fuel and timing to move the vehicle to a repair facility.  Believe it or not, it's programmed that way intentionally to force repair.

[Irish_Alley]

bd i thought when the o2 fails or is in-op then the ecm goes into open mode which gives the engine more fuel than normal. and this is why its running good when cold but once warmed up it still has too much fuel causing it to run rich. but running rich problem can seam like its running lean problem right? but yeah first things first either mount your o2 sensor or buy a dummy o2 sensor. the o2 sensors not the only thing that will cause this problem but since you know its not there its a good place to start

[bd]

bd i thought when the o2 fails or is in-op then the ecm goes into open mode which gives the engine more fuel than normal. and this is why its running good when cold but once warmed up it still has too much fuel causing it to run rich. but running rich problem can seam like its running lean problem right? but yeah first things first either mount your o2 sensor or buy a dummy o2 sensor. the o2 sensors not the only thing that will cause this problem but since you know its not there its a good place to start

When an OBD-I ECM receives no input from select sensors (or input that is grossly out of range) after a set period of time, until the engine is cycled OFF, it will default into "Limp Mode," which bypasses the offending sensor and "fixes" the sensor circuit at a predetermined voltage.  The engine will run, but generally horribly and vehicle speed may be severely limited.  The factory did this so major components such as the engine and transmission wouldn't be damaged while the vehicle and its occupants wouldn't be stranded.  Limp mode results when the ECM registers a gross anomaly in its logic.

When the O₂ sensor remains in open loop long enough after the engine has heated to operating temperature, with the ECM receiving variable input from other sensors, including the vehicle speed buffer and distributor tach, the ECM will record a code, bypass the O₂ sensor and substitute an average internal voltage.  Depending on circumstances, save for an illuminated SES light, the driver may not even notice there's a problem.  However, in a severe case where the O₂ sensor is missing from the circuit entirely, the ECM 'may' default into Limp Mode, causing the engine to run exceedingly poor once it warms up.

[Irish_Alley]

why you say "may" why not "the ecm will default into limp mode"? what might cause the ecm to stay in open or change to limp?

[Irish_Alley]

afaik limp mode affects the trans, tcc is disabled you only have R and 3rd to get 2nd you have to manually shift and this help you "limp" home only going 30mph in 3rd gear. having a problem that ONLY occurs when hot seams like a running rich problem and this can cause more problems with cats and all. but if you tried to "limp" home with the ecm doing this it would cost more to fix the problems it can create

[johnnyr1965]

Thanks for all the input.I will install a O2 sensor and let yall know.Hope that fixes problem.

[johnnyr1965]

My 87 with 350 TBI does not have a O2 sensor. I have welded a bung in and installed the sensor.My question is where do the wires lead to.There were no wires hanging down.Do they go to the ECM or the distribator?