1990 Chevy C1500 TBI/305 stock

[zieg85]

I am trying to troubleshoot a truck I just bought as a daily driver...

It is a 1990 C1500 with a 5.0L 700R4.  It runs great until the engine is completely warm then it misses especially under load (climbing a slight hill).  I just replaced the cap and rotor.  A couple days ago I replaced the O2 sensor (rusted out and hanging by the wire) then the coolant temp sensor, code 14 check engine light which now I have no light.  What else could cause this?  TPS/MAP/EGR? Again it runs great until fully warm, about 15-20 minutes, temps have been high 80's

[VileZambonie]

How are the plugs and wires? Check the exhaust back pressure.

[zieg85]

How are the plugs and wires? Check the exhaust back pressure.

Plugs look great, beige colored nothing fouled.  Plug wires OK, no burn marks.  No back pressure, original cat, turbo muffler right after.  I don't understand the TBI and how the sensors start working once operating temps are met.  It runs like a scalded dog and when the operating temp gauge hit the mark it is like a switch turns on and it starts to miss.  It doesn't set a code.  What sensors would be inactive until that time?

[zieg85]

Truck only has 102,300 and no blow by or signs of wear.  It doesn't leak anything either

[dusterdude]

Unplug the o2 sensor and drive it,if it runs good after it gets warm you have a closed loop issue with the system.

Sent from my LM-X420 using Tapatalk

[VileZambonie]

Do you have datastream on a scan tool? What's the fuel integrator & block learn at? Have you checked for vacuum leaks?

[zieg85]

using the chart pictured I measured 286 ohms at 184 degrees water temp.  I will unplug the O2 sensor and take it for a spin next. 

[zieg85]

Unplug the o2 sensor and drive it,if it runs good after it gets warm you have a closed loop issue with the system.

Sent from my LM-X420 using Tapatalk

Runs the same as it did plugged in when at operating temp

[zieg85]

Do you have datastream on a scan tool? What's the fuel integrator & block learn at? Have you checked for vacuum leaks?

I only have the mid level OBD2 Craftsman 87702 scanner.  No vacuum leaks that I can detect.  Nice and quiet at idle and guessing 600 rpm, 90's didn't have Tach's

[zieg85]

Do you have datastream on a scan tool? What's the fuel integrator & block learn at? Have you checked for vacuum leaks?

Do these cables actually work?

https://www.ebay.com/itm/GM-12-Pin-OBD1-to-16-Pin-OBD2-Convertor-Adapter-Cable-for-Diagnostic-Scanner-New-/401533946179

[MIKE S]

Look at the main shaft in the distributor. Does it have any side to side movement. Also check the magnets at the top for cracks. If you find you need to replace the distributor get a new one not a rebuilt.

[TexasRed]

May be the ignition coil? It gets hot and starts layering short?

[VileZambonie]

Do you have datastream on a scan tool? What's the fuel integrator & block learn at? Have you checked for vacuum leaks?

Do these cables actually work?

https://www.ebay.com/itm/GM-12-Pin-OBD1-to-16-Pin-OBD2-Convertor-Adapter-Cable-for-Diagnostic-Scanner-New-/401533946179

For $8 I'm sure it's worth a try but if oyu are going to own an OBDI vehicle, you need to have a scan tool for it.

[zieg85]

I had a slight bit of play in the distributor so I changed that and returned the new ignition module I didn't use.  That wasn't it.  I put a new coil on it, no dice either.  Next logical thing was to just replace the spark plugs.  That was it.  No visible cracks, all looked great, proper gap but obviously were failing.  Thanks so much for the suggestions.

[VileZambonie]

How are the plugs and wires? Check the exhaust back pressure.

Always start with the basics. Definitely get yourself and OBDI scan tool