Surging when heater turned on

1993 k3500 7.4 tbi nv4500.
Truck will surge than almost stall and repeat when heater is turned on. I recently had to replace the lower radiator hose. Since I had to drain most of the coolant anyways I thought it be a good time to replace thermostat and cts. The cts is the cheap oreillys brand which I read could be an issue. Problem is I just got the truck and never turned on the heater before so I'm not sure if it had the problem before I installed the new cts and thermostat. At first it would throw dtc 35 intermittently so I put a new IAC which didn't fix it. Sometimes the check engine light will come on briefly and when it does the truck seems to run better. It gets better as the truck warms up but never fully goes away. Also seems like it takes a long time to warm up. Have been trying to find anyone experiencing this same problem but have yet to find anything.

jimmyd said:

1993 k3500 7.4 tbi nv4500.
Truck will surge than almost stall and repeat when heater is turned on.

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What do you mean "heater is turned on"?

- the temp selector is set to something other than "cool"

- the blower switch is set to a certain setting(s)

- (enter your complete and detailed description here)

If I had to guess, you've got an electrical connection(s) that's compromised.

With that thought in mind, check the condition / function of all the ground connections...

- ECU ground connections on the engine intake manifold

- body / frame / engine ground straps, found behind the engine on the passenger's side, sort-of between the exhaust manifold and the body

jimmyd said:

I recently had to replace the lower radiator hose. Since I had to drain most of the coolant anyways I thought it be a good time to replace thermostat and cts. The cts is the cheap oreillys brand which I read could be an issue. Problem is I just got the truck and never turned on the heater before so I'm not sure if it had the problem before I installed the new cts and thermostat.

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^^^ For now, I don't think the CTS or other details you mentioned are related to your problem.

jimmyd said:

At first it would throw dtc 35 intermittently so I put a new IAC which didn't fix it. Sometimes the check engine light will come on briefly and when it does the truck seems to run better. It gets better as the truck warms up but never fully goes away.

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^^^ Get a scan tool, check for all codes, and report what you find in detail. Include the operating conditions / engine temp / engine run time and other relevant details.

DTC 35 might be caused by a vacuum leak, e.g., a broken/leaky vacuum hose, leakage past the throttle shaft in the TBI, or elsewhere. Others may have more to say on this.

jimmyd said:

Also seems like it takes a long time to warm up.

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^^^ Get a scan tool and see what engine temperature the ECU is reporting. Don't trust the gauge on the dash. What's important is what the ECU is reading for coolant temp.

If your gauge isn't showing the proper temperature, you can worry about that later. The gauge receives it's coolant temp signal from a different sender IIRC.

Coolant temp sensor for the water temp gauge or ECM ? Just to be clear.
Did you check the IAC before install and the do the relearn procedure.

jimmyd said:

1993 k3500 7.4 tbi nv4500.

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jimmyd said:

Truck will surge than almost stall and repeat when heater is turned on.

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HEATER, or "Defroster"?

If you mean "Defroster", using that position engages the A/C compressor, which adds additional load to the engine.

jimmyd said:

it would throw dtc 35 intermittently so I put a new IAC which didn't fix it.

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Download the service manual set for your vehicle from the links in the Sticky thread section of the Engine forum.

Follow the diagnostic procedure for Code 35.

jimmyd said:

Sometimes the check engine light will come on briefly and when it does the truck seems to run better. It gets better as the truck warms up but never fully goes away.

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Those are clues.

jimmyd said:

Also seems like it takes a long time to warm up.

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Defective new thermostat? Incorrect thermostat?




The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)
   
2. How old are the usual “tune-up” parts and procedures? Inspect/replace distributor cap, rotor, plug wires, spark plugs. Make sure the ignition coil will reliably fire a spark-tester calibrated for HEI when the coil is fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper initial timing (TBI) and electronic spark advance (TBI and Vortec). Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.
   
3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. Look for misfire counts for each cylinder (OBD2 only.) “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle. The service manual set can be downloaded from the links in the Sticky thread section of the Engine forum on this web site.