Low/fluctuating Idle and No Start

[Antichrist]

Just an update on the situation so far (I hated finding threads about problems that nobody posts their results in.. not helpful when you find them in searches)

I unplugged the oil switch sensor, and the truck started as usual. I will still have to address the oil leak in the near future though.

Oil pressure when warm seems to be in the correct operating range, 40ish...

I realized the truck has two sensors for coolant. One in the head, for the guage. And one in the manifold by the tstat housing, for the ECM.

There is also a bolt with a handful of ground wires, and electrical tape there. Add to that, the corroded and rusted ground point, and I might have located the problem.

The truck behaved the same today as before. Longer cranks to start, idled rough after a few minutes in gear, etc..

I unplugged the coolant sensor, the idled surged, then leveled off. CEL came on, code 15 for low/high voltage, I forgot which, the book is in the truck right now.

I cleared the codes, plugged the sensor back in, started the truck, it fired up perfectly. I put it in gear, and I'm getting idle problems INSTANTLY. Before I had to wait 2-3 minutes or so.

I repeated the test with the truck running, and in park. Unplugged CTS. Same problem, CEL came on, idle surge, then levels off. Shift to drive it wants to stall.

Plugged CTS back in with truck running, idle didn't change. CEL went out. Shift to drive, and it immediately runs rough and wants to stall.

After the engine cools, I'm going to check the harness for shorts to ground, I'll redo the electrical tape nightmare and reground the wires by the CTS properly, and I'll see what happens. If I have the same symptoms, I may be replacing the CTS and/or harness, depending on what I find.

And I'll take a look under the truck to see what's going on with that random hose on the bellhousing shortly.

EDIT: The hose that I found on the bellhousing runs back along some wire loom back to the transfercase.

I cleaned up the grounds, found one ground that had broken off from the ring terminal. sanded all grounding surfaces to expose new metal, cleaned and sanded all ring terminals, reconnected everything.

The truck started better than usual, took it out for a 70mph drive, I was even able to get a little wheel spin, so it had more power than it did before, when I got home I put it in drive, sat with my foot on the brake, and shortly after, the idle problem returned.

During this drive, the temperature guage never exceeded 195-200.

EDIT (again): I found a new possible issue, I believe my thermostat might be sticking closed, or at least partially closed.. The upper hose gets hot, but when the truck is running, I can easily squeeze the upper hose the same way I can when the truck is off.

I suspect my tstat might be sticking partially closed, so for $5 I'm going to replace that now, and see what happens after that.

 

[SunlitComet]

If t-case vent would not expect the t-case vent line to be open like that. Thought there was supposed to be a vented cover on it. Better yet mounted else under chassis. COuld be wrong.

 

[Antichrist]

I agree 100%. I assume it should be vented in a way similar to the rear differential fitting that's in the hose. Just another thing for me to fix.

I replaced the thermostat with a 195 degree tstat, which the existing one was also 195 deg. No change in hose pressure (Maybe I'm expecting the hose to be too firm?) And no change in the idle problem. One of the bolts also partially stripped the manifold, but a longer bolt worked fine for now. I'll use this as an excuse to buy a few performance goodies in the spring.

I'm pulling code 33, for a bad MAP, but I read that a poor idle can also cause this code. And my MAP sensor is brand new.

I idled the truck in park, pulled the vac line from the front of the EGR solenoid (the line directly from the TB that goes INTO the solenoid) And I have great vacuum.

I pulled the line off the back of the solenoid, that goes to the EGR, and I can't feel a thing. Unplugged, and replugged the wire harness, still no vacuum. I'm going to test this one more time with my girlfriend in the truck, wait until it starts idling rough, then pull some hoses/harnesses and see what happens.

But right now I'm thinking this is either the cause of my problem, or a contributing factor..

How concerned should I be with the upper radiator hose pressure, of lack therof? I have great heat inside the truck. I don't remember the temp guage ever going past 195 or 200. The truck shows no signs of overheating, or not running hot enough for that matter.. I just want to make sure I don't have an issue with the impellers inside the water pump. I'd rather fix any major issues now before the snow hits.

I'll post another update when/if I find anything else. Hopefully this thread will help someone else in the future, and possibly eliminate them going through the same headaches I've been dealing with for about 2 weeks now..

 

[SunlitComet]

Improper pressure cap or broken cap installed? should be a 15 pounder. If you are running alot colder then normal it also could be a sign that cylinders are not firing creating the heat you need to extract. But when I blew out Perhaps have the rad cap and compression tested if you can. Compression testing can give you an overall outlook on mechanical condition of each cylinder.

 

[Antichrist]

My parents have a 1997 Tahoe, with the Vortec 5.7, and spider injection system. I've done almost all of their maintenance so when I bought my truck I was already familiar with how a properlay maintained Tahoe should feel. My truck runs, heats up, drives, and accelerates just as well as theirs.

I'll have to wait until this weekend to go swap radiator caps with them and see if I have any change in pressure. As far as compression, I don't have any noticable issues with a lack of heat. My truck actually heats up better than a few other GM cars and trucks that I've owned previously.

I'll check compression for the hell of it this weekend if I have time and cooperating weather.

I'm starting to suspect an EGR solenoid, or valve problem at this point in time. I'm going to remove both and make sure the solenoid isn't allowing vaccum to leak past somehow, as well as make sure that the valve isn't sticking open. I already noticed that moving the diaphragm feels like there's some resistance on the rod that guides the diaphragm, I' assuming if this doesn't close all the way it could cause an issue as well.

I started learning about cars and helping my father when I was maybe 12 or 13.. working on them since about 18. I'm 31 now, and this is by far the most difficult problem to diagnose that I have ever encountered.

Ah well.. Thanks again for all the input. I should know more about the EGR system either today or tomorrow.

 

[JonnyTahoe]

When in doubt call jonny T. I have the same year truck and have had it for 5 years and have had a EGR code issue for years. Did I replace it, NO The truck runs just fine and always has so move on to something else. I have always had a rich fuel condition that I was concerned about and thought it was because of a faulty thermostat making the truck run in cold start mode so I replaced the thermostat 3 different times, did it fix the problem, NO. That hose you took a pic of I need a better pic to see which one it is but for now Plug it and Move on. The problem seems to be in the fuel system not the oil pressure,EGR, or cooling system. Take that Thottle Body off and clean every inch of it and check both injectors for proper operation. Each injector comes apart and can be cleaned. Before you take the truck out for another "Test Drive" pull that Big Chamber air tube thingy off if you have not already done so. I take the entire Air Cleaner assembly off that way I can "see" everything. You might want to Blast the motor now at the car wash so in the middle of winter when your intake gasket starts leaking your motor will be nice and clean to work on. JT

 

[Antichrist]

After running the truck and refilling the coolant, I can now feel definite hose pressure. I believe I just had air in the lines that wasn't properly bled out by the previous owner. Upper hose gets warm now, it didn't previously. I probably have more air in the system, but we're getting better.

Following information I found in random google rasults for similar symptoms, I checked my injectors with my timing light, running from the coil wire, and no issues, pulses are dead on. To check for dripping injectors, I unplugged them, one at a time, and each time the truck stalled.

To test EGR solenoid, I ran a vacuum line across the engine so I could operate the throttle and feel for vacuum from the solenoid at the same time.

At idle I have no vacuum, so it's not opening the EGR at low idle. I gave it a little quick throttle and let off, and felt some vacuum, I then held RPMs at about 3k, and felt the vacuum, and it quickly bled off. After about 1-2 seconds there was absolutely no vacuum.

Still haven't gotten to the radiator cap, or compression test. It's getting cold here, and the constant leaf raking is taking up a lot of my time.

Jonny: I don't have any EGR codes. The hose in the picture was found to be the t-case vent. It's just missing the vent cap.
I have a hard time understanding it being fuel related. I have long crank times (5 sec or so), and longer in cold weather. In drive while stopped the truck wants to stall. Shift to park or neutral (same RPM range) and the truck runs fine. I don't understand how the fuel pump, fuel filter, injectors, and/or regulator will have an issue maintaining pressure ONLY when I'm stopped at a red light while the truck is in gear. If I shift, the problem disappears... It doesn't make sense that my fuel system is more than adequate to maintain a flawless 65 mph speed, but stumbles at red lights.

If I'm wrong about the fuel system, and it does in fact require higher pressure while in gear and stopped, than it does in park or neutral, please correct me.

 

[SunlitComet]

These engines do not require a specified bleed procedure. After filling cooling system, run engine to operating temperature with radiatorhttp://www.alldatadiy.com/alldata/A...62806635/34853741/100411974/34856156/34856286/pressure cap off. Air will then be automatically bled through cap opening. minute amounts will bled on there own thru reservoir over a few hot cold engine cycles.

 

[Antichrist]

Thanks for the tip I haven't driven the truck much while I'm ironing out these issues, so I'm guessing the fact that the coolant was low when I got the truck was just the main issue.

I was just out for a drive, 30 minutes one way. I noticed something that may or may not be related to my other problem.

When I accelerate from a stop normal/hard It runs great. When I'm holding speed it runs fine, when I activate OD, it downshifts and runs fine.

But when I'm cruising at a steady speed, and it does this at all speeds, when I apply LIGHT acceleration, it's really sluggish.

So right now I'm unsure if the EGR solenoid is operating correctly, as it doesn't hold vacuum. It gives a single shot, then bleeds off.

The coolant temp sens plastic part spins inside the metal fitting. When I unplugged it ysterday with the truck warm, I simulated a cold start condition, and the truck would not start. Is this normal?

I noticed my stalling in gear and stopped, occurs over 145 degrees.

And I read elsewhere that similar symptoms of the sluggishness were caused by a broken spring inside the regulator. Any thoughts?

 

[SunlitComet]

coolant switch issue is not normal to my knowledge and should be replaced. And if you are sure the egr is not holding when testing with a vacuum pump directly then replace it. Recheck you vehicle for any vacuum leaks as well and report back here please.