Diesel Engine Hard Start Temperatures Below Freezing - keywords additive cold filter freeze fuel gel gelling LBZ LB7 LLY no P0087 P0093 P1093 restriction temp temperature vacuum wax waxing

This PI was revised to include the 2007 model year. Please discard PIP3317D.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

A 6.6 LBZ LB7 or LLY diesel engine equipped vehicle may be brought in for a hard start, or a no start only when outside temperatures are below freezing.

The customer may state the vehicle started, ran for a short period of time and then stalled out.

If the vehicle is moved into a heated shop or garage the vehicle will start normally.

If a below freezing hard start/no start has been verified it may be caused by fuel contamination, poor fuel quality, fuel gelling, or fuel waxing.

Open the fuel filter drain and get a large fuel sample out of the fuel filter. If water is found in the fuel system it may be the cause of the no start or hard start condition. The water in the fuel tank will freeze and restrict the fuel flow. During the hard start/no start condition monitor the fuel system vacuum readings to diagnose the concern. Normal fuel system readings when cranking are 1 to 2 inches Hg (in/Hg) of vacuum. A frozen restricted fuel system can reach 10 in/Hg of vacuum or higher while cranking.

If no water is found in the fuel system check the fuel API reading with the Kent-Moore J38641-B. The fuel API on winter blend fuels is approximately 35 to 44 depending on the location of the vehicle. Fuel APIs below 35 to 44 (summer blend) would have a higher possibility of waxing. Monitor the fuel system vacuum readings to diagnose the concern. Normal fuel system readings when cranking are 1 to 2 in/Hg of vacuum. A waxed/gelled fuel system can reach 10 in/Hg of vacuum or higher while cranking.

Below is a test to help pinpoint the area of restriction:

1. Disconnect the fuel supply line at the quick connect over the left valve cover.
2. Install a hand operated vacuum pump (Kent-Moore J23738-A) to the chassis side fuel supply line.
3. Apply vacuum on the line in an effort to draw fuel from the fuel tank.
4. Note the vacuum required to pull fuel up through the fuel line.
5. If the vacuum is greater than 5 in/Hg (typically 15 to 20 in/Hg) to pull fuel from the tank then the restriction is either in the chassis lines or fuel tank pickup.
6. If the vacuum required to pull fuel from the fuel tank is lower than 5 in/Hg then the issue is on the engine side of the fuel system. In this case follow published fuel system diagnostics in SI.

Note: This procedure must be completed in a timely manner (less than 50 min.) to find the root cause.

Note: Check the vehicle build date in GMVIS. If the vehicle was built between 1/21/2005 and 1/31/2005 the tank must be removed and inspected for signs of water. Vehicles built between these dates may have a slight amount of water left in the fuel tank from the fuel tank pressure testing procedures. Complete current SI procedures for Fuel System Cleaning if water is found.

Bulletin Number 03-06-04-017C describes the use of fuel additives for water removal and cold weather operation. If large amounts of water or contamination are found in the fuel system follow the current SI procedures for Fuel System Cleaning.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.