Starts in the morning and just cranks in the afternoon

[SierraMike]

So my 1998 Sierra 5.7 started having ignition issues. First start of the day it fires right up. After work in the afternoon and first crank it tries to catch but then just cranks and won't catch. After multiple attempts it will eventually start. I'm roaming the internet for ideas, any help would be greatly appreciated.

Thanks for reading.

Mike

Edited July 31, 2021 by SierraMike

[Jsdirt]

Are you sure it's ignition and not fuel? That screams of low fuel pressure, or excessive bleed-down to me.

[SierraMike]

Not totally sure, fuel pressure is about 55 psi which is a little low I think. I guess probably fuel pump eh? Or check valve? 

Thanks for responding btw.

[Jsdirt]

You're welcome.

 

Should run on 55, but I wonder if it's immediately bleeding down to zero PSI once the key is off ...

 

How many miles on her? Might just need a tune up. Plugs, wires, air filter, cap and rotor if it has one, etc ...

Edited July 31, 2021 by Jsdirt

[SierraMike]

I think you may be right, she has 167,000 miles and I've put half that on myself so maybe cap and rotor, plugs and wires wouldn't hurt.  Last few days its been cooler and the truck seems to want to start in the afternoon a little easier. Not sure why ambient temperature would play a roll. Anyways thanks for the advice

[Jsdirt]

Lower temps = lower resistance. Starting to sound more like worn or failing ignition components, unless the mixture is richer than it should be due to a leaky injector or 2, or a failing pressure regulator. Lower temps also mean a leaner mixture for a given amount of fuel. The ECU should be able to adjust for that, but if the o2's or MAF, or temp. sensor are off a bit then that's plausible. Usually the light will come on, but it could be right on the borderline of triggering it.

 

Can be tough sometimes to differentiate between an ignition and a fuel problem. Both can exhibit similar symptoms. I'd still do a tune up and go from there. It's definitely going to help with starting if nothing's been replaced in a while.

[SierraMike]

I know I have a bad o2 sensor but I cant get it out as its rusted in. Someone else has suggested a leaking injector as well although I don't understand why the truck would fire right up in the morning if a cylinder was full of fuel. Plus I would think the fuel would get past the piston ring overnight and increase the oil level. I did wonder if it's an intake issue as well, I pulled the air box off the throttle body and everything looked fine. I guess I will have to pick a place to start, throw parts at it and just eliminate the possibilities one at a time.

 

You've been a great help, thank you for sharing your knowledge.

[Jsdirt]

You're welcome.

 

Engines need a richer mixture when cold - colder they are, the richer they need to be. That's why the coolant temp sensor is so important.

 

You won't have a cylinder full of fuel with a leaky injector - once the key is off, the pump is off, and no longer making pressure. The injector will dribble down until there is zero psi, and that's the end of it. Firing it up in the morning, that extra fuel, which isn't much, sitting on top of the piston provides a more perfect mixture the engine needs to run. Once hot, the tables turn - you require a leaner mixture with temperature increases. Now it's going to crank and crank until that extra fuel is purged out and it becomes lean enough to fire.

 

That o2 sensor could very well be the problem if it is an upstream (before the catalytic converter) sensor.

 

The other thing I mentioned earlier is still a possibility - a bad regulator allowing pressure to bleed back to the pump when the key is off. Could be that it works once in the morning, then craps out later after it's been used. If your pressure drops straight to zero PSI fairly quickly when the key is switched off from idle, this will also cause a long crank time. I would do the fuel pressure test after you've driven it, and everything is hot. Makes it sucky to screw the fitting on, but might provide a more accurate picture of what's happening here. Keep in mind, a real bad injector can bleed the system down the exact same way.

Edited August 10, 2021 by Jsdirt

[SierraMike]

That's great food for thought. Interesting development today, doesn't want to hold idle and stalls. Also the clock resets to 1:00.  So after it fires I hold the throttle a bit to keep it going and then start driving and it bogs out when the throttle is pressed. After a short distance it seems to move past the problem and throttle response returns to normal.....

[Jsdirt]

I'd check your battery terminals, cables, and the other ends of them both. Might just have a loose connection somewhere. That would certainly make the computer flake out and cause drivability issues. 

 

Sometimes just a thorough visual inspection will turn up an issue. Check all around the engine compartment for wires rubbing against metal, or hot exhaust. After 23 years of bouncing down the road, anything is possible. I'd crawl underneath as well and check everything under there, especially grounds. I've seen plenty of those flat woven brass ground straps  rotted right in half under stuff from the 90's. They usually put those from body to frame.

 

A failing fuel pump will also cause these symptoms. You'd have to tape a fuel pressure gauge to your windshield in the morning and see if it's dipping low during the stalling / rough idling. Watching a scan tool's live data will give you much better direction. If both upstream o2s go lean, followed by the downstream ones, you can bet you're losing fuel pressure / volume.

[SierraMike]

Thankfully ( and embarrassingly ) it seems to have been a loose positive connection at the battery.  Its a stupid side terminal one and I've stripped the lead threads in the past so I guess this time i didn't tighten it enough. Very grateful for your help and time, I've learned a lot, hopefully I can repay the favor by trying to help someone else in need.

Cheers!

[Jsdirt]

Glad it was an cheap AND easy fix!! Best outcome there is in the automotive world.

 

Happy to help.