1994 Yukon cold starting problem

[dragonriot]

Whenever my truck sits for more than say 5-6 hours, I consider it a cold start... I turn the key to on, let the glow plugs cycle, then crank. 50% of the time, the truck will start on the first crank, and within 3-5 seconds. The other 50% however, it doesn't start on the first crank, so I immediately turn off/turn the key back to on, and let the glow plugs cycle again, which usually only takes about a second or two. I crank, and no fire... this morning it took me about 2 minutes to get the truck started, that was 15 seconds of cranking/glow plug firing, 1 minute cool down, and tried again... it almost always fires up after the minute wait, but I don't understand why it's not starting only 50% of the time.

Any help would be appreciated.

[john8662]

Is the check engine light on when driving?

Verify it works too! (should illuminate before starting)...

Hard starting is usually due to a couple of common things.

Burned out glowplugs, recommend testing them and going from there, They usually do not all fail at once. You can test these with a test light, remove the connector and then test individually, if you get the light to work, then the plug is likely good.

Fuel supply problem.

Check and replace the fuel filter.

Check the fuel lift pump operation. Do you hear the pump run when you turn the key to run? It should turn on before cranktime. If yours it not very loud, you might have someone turn the key while you feel the pump pulsate below (it's located on the driver's side frame rail).

Verify there is no air in the system, or fuel leaks (do you smell fuel).

J

[dragonriot]

Is the check engine light on when driving?
Yes, intermittently, diagnosed as Accelerator Pedal Positioning Sensor #1 being out of sync... shouldn't affect starting, since the pedal position has no affect on a computer controlled engine start.

Verify it works too! (should illuminate before starting)...
It works...

Hard starting is usually due to a couple of common things.
Burned out glowplugs, recommend testing them and going from there, They usually do not all fail at once. You can test these with a test light, remove the connector and then test individually, if you get the light to work, then the plug is likely good.
Haven't checked it because I haven't had time yet... I've owned the truck for about 3 weeks. At first I thought it was just being finicky... then it started doing it more often. I'll do this check as soon as possible.

Fuel supply problem.
Check and replace the fuel filter.
I just had an inspection done, and the fuel filter is on my list of things to replace. The truck runs great after it starts though, so I don't know how much of an effect the filter could be having on the startup.

Check the fuel lift pump operation. Do you hear the pump run when you turn the key to run? It should turn on before cranktime. If yours it not very loud, you might have someone turn the key while you feel the pump pulsate below (it's located on the driver's side frame rail).

Yup, I can hear the pump. I almost always stand outside the truck when starting it, so I hear it every time.

Verify there is no air in the system, or fuel leaks (do you smell fuel).
J

How do I test for air in the system? There are no fuel leaks, and I never smell fuel. When I start the truck, I get a single white cloud of smoke, then no smoke at all. If I give it full throttle, I get black smoke, and I have noticed a sooty buildup on the body just above the exhaust pipe. I'm going in tomorrow to have my Sway bar links replaced, so I'll call the shop now and get my K&N filter cleaning kit and Fuel Filter ordered today. Maybe we'll have time to check the glow plugs while it's there.

By the way, thanks for the checklist... =)

[dragonriot]

Well... turns out I wasn't hearing the lift pump... in fact, it wasn't running at all because there is no power to the lift pump fuse. The PO decided to hardwire the lift pump into the fuse block in the cab, and still it wasn't actually working. We moved the wire to an "always hot" socket on the fuse block, so that when I turn the key to the On position, the lift pump runs constantly. The problem now, is that the thing won't shut off while the truck is running. Whether I'm at idle or full throttle, I can hear the incessant vibration of the pump.

So I now have a problem that requires immediate attention. I stopped at Diesel Injection Service today, but they didn't have time to look at it. My two previous tanks of fuel, I got 18MPG average per tank for combined city and highway driving with 265/75R16s and 4.11 gears. I topped off the tank today because I was going on a 2 1/2 hour drive, and did my mileage calculations. I put in 13.466 gallons of B5, and had 200.4 miles on the trip odometer, which comes out to around 14MPG. I had 100 miles on the clock before I did glowplug and filter service yesterday, and put on another 100 miles afterwards. That means after my service, I lost at least 6 if not 8MPG in the second 100 miles I put on the clock. Now, I know I can't really judge a whole 500+mile per tank average on just 200 miles, but I'm worried.

Yesterday's service included new Ac Delco GM-60G Glowplugs, new Wix Air filter and new Wix Fuel filter. Non-engine service was limited to my sway bar links. I also pulled the wiper motor control board and looked for a bad solder joint, but found none so I adjusted the contact arms and smeared a little bit more grease around in the wiper control box. Along with my seemingly poor mileage - I can still "watch" my fuel gauge move while I'm driving - the truck is still starting hard when it's cold, and my exhaust is abnormally sooty.

The DIS service advisor said my lift pump is probably over-feeding the injectors, and instead of sending all the unused fuel back to the return line, the injectors spray too much, and that's where I'm losing my mileage - and it's also causing a "rich" condition so my exhaust is quite thick. The white paint of the truck and the chrome on my bumper looks like it was spray painted flat black.

I would greatly appreciate any suggestions or help you guys can give me, because I'll be driving a good 500 miles just this weekend, and getting 14MPG unloaded is just not going to cut it. Please consider this an emergency. If you have suggestions that can't be expressed through typed words, my phone number will be listed in my personal profile. Thanks in advance.

***EDIT*** Oil pressure is always good at 40-50PSI, tires are fully inflated to 40PSI, I am not abnormally loaded with any amount of weight that would cause this much mileage loss... Clarification... the Lift pump fuse is not actually blown... just not providing power to the pump. It also didn't work when we shorted the jumpers with the test light, but it does work when it's hard wired.

[Robyn]

OK
First off the lift pump will not affect mileage. The fuel is injected into the engine by the injector pump and the injectors are simply a nozzle.
If you have as was previously stated an issue with the TPS you need to address this.
The computer may very well be running in cripple mode and this well could account for the poor mileage.
The hard starting issue can be one of several things.
Is the starter spinning the engine real good, if not check batteries and cables
Well worn Injector nozzles can contribute greatly to poor cold starts.
The nozzles get sticky and carboned up and will pop off early and at too low of a pressure resulting in a stream of fuel as opposed to a fine mist.
The lift pump is wired through two independent circuits, the first routes power from the start circuit that runs the starter solenoid to the lift pump during cranking to get fuel pressure up for the start. Once the engine is runing the oil pressure switch has a circuit that keeps power on the pump constantly.
YES these pumps run all the time, they are supposed to.
The lift pump has a 20 amp fuse located in the junction box on the firewall by the AC accumulator. Also there is a fuel pump relay that is in that
same junction box.
Poor mileage is probably being caused by worn out Nozzles and possibly the issue with the TPS.
You need to trace out and repair the issue with the fuel pump (Lift)
Get a couple of the Injector nozzles out and have them tested for pop pressure and pattern.
If the nozzles are in poor shape you need to replace them with a fresh set.
Several advertisers here at the page can help with this or your local diesel rebuild shop can too.
With your newness to this rig you really dont have any track record and the previous owner sold it for a reason.
Disconnect the batteries for a couple minutes to clear the codes and rerun the engine and get a good reading on any and all codes. If the TPS is the only one replace the little beast. Its 3 screws and a wiring harness that plugs in. A no brainer. You dont have to reprogram anything, the rig will work fine with a new one. These are not cheap so if you can find a "KNOWN GOOD ONE" maybe a way to go.
Another thing, A yukon is a 1/2 ton rig like a full sized Blazer used to be.
It has an EGR system, make sure its working properly, a failure of this sytem resulting in EGR at the wrong time can mess mileage up big time.
The EGR is in the center of the air inlet. Also make sure your turbo is working, check that you have a minimum of 25 inches of vacuum at the wastegate at idle. The vacuum should be strong and steady, if its poor or the vacuum gauge wobbles around replace the vacuum pump. The waste gate solenoid can be bad and not toss a code. (located on drivers side valve cover near the back)
Many times the vacuum lines break and folks just dont catch it. The rig will run but power will suffer and the exhaust will be very sooty. The waste gate vacuum actuator can also fail. Another check is to be sure the waste gate is working, I have seen them stick. With the rig off check that the waste gate rod can be moved back and forth easily. When the rig is running you should not be able to move it by hand. Proper vacuum is very important.
All this stuff is very basic and can be fixed easily with a few $$$$ and a little time. GET OUT THE TOOLS. :0)

Keep us posted
Robyn