Stressed with an engine problem..

[98_2DR]

Hello guys and gals, I'm going to start off with thanks for everything you guys have already given me, and hope we can figure this out together!!

I have a 98 Tahoe 2dr with the 350 vortec 5.7l engine and it is running rough, and consuming waaaay more fuel than it used to. I drive mostly highway and I have been getting about 390-400 miles from a full tank now I'm getting MAYBE 200. It is not throwing any codes, I use real fuel not ethanol, I use chevron fuel system cleaner every 4th tank of gas. I have ac delco iridium plugs in her which are about 2 months old, a cheapo rotor and cap from eBay. The rotor and cap was replaced a month ago. Ran fine until about two weeks ago started running rough and using to much gas. Took to autozone to have them read any stored or pending codes and it came up clean. I'm lost as to what to do.

All the new parts are below.

New oem fuel pump and sending unit
New ac delco plugs
New thermostat
New cap and rotor
New fuel filter

Please help!

 

[lesterl]

Is it hard to warm start? Wonder if you have a vac leak or a leaking FPR or something like that.

 

[98_2DR]

No it's never hard to start. Starts right up every time. Some times it does turn over a time or two like all my other cars, and sometimes it seems like it starts right up but it doesn't even turn over once (hope that makes sense).

I did a visual on the plugs wires and vacuum hoses and everything looks to be in order but clearly it's not, it's costing me $20 in fuel every day and a half. I need this fixed..

What is a fpr?

 

[lesterl]

FPR is Fuel Pressure Regulator, this is inside the intake manifold and would dump fuel into the engine, were any of the plugs black/sooty or wet?

 

[98_2DR]

The very back plug on passenger side was wet, as the plug are pretty new they still looked pretty clean.

Is this a hard part to replace? You say inside the intake manifold, do I have to pull the intake manifold to get to it?

 

[Beeker702]

You would have to pull the upper intake it that's the case. What's your plug wires look like? And your cheapo cap and rotor could cause your problem as well

 

[lesterl]

Do a fuel pressure leakdown test...

 

[samotahoe]

WAIT! here is an easy test to do before you start tearing everything down. if you have a compatable scanner or digital multimeter you should check the o2 sensors. watch the fuel trim and the o2 sensor could be biased. see if the signal voltage crosses 450mv and if the primary hangs out too long above the 450mv or lower than the 450mv range it could be sending bad signal to pcm causing the rich condition. especially if you have exhaust leak. also see if you have any negative voltage on the o2 sensors then they are automatically trash!

 

[cj97tahoe]

Also try cleaning your MAF meter. Gave me an improvement.

 

[98_2DR]

Took Tahoe into a shop today and the ran every test they could and hooking it to their computer and they said only things stored in the history was abs related but was just in history and not to worry, no misfires, no drivability issues nothing... They said use a hotter thermostat or take to a muffler shop and have the cats tested, they blew a hole into the pipes and they told me it passed everything. I'm lost...

I was told that since the truck has 201,000 miles on the truck it's way over due for cats... Did not know they wear out. I know they get plugged stuff but wear out? The shop said twill be best to just cut out cats weld in strait pipes and reprogram the computer to ignore the o2 sensors, do something for the fuel mileage and change shift points... This will cost $250 for all of that or $600 to replace cats.

 

[98_2DR]

Also today I cleaned maf sensor with the maf spray cleaner, cleaned throttle body with the proper cleaner. Checked all my plug wires again, and listens for vacuum leaks. While truck was running I sprayed throttle body cleaner around the lines and intake and it did not idle any different, nor did it rev up like I have been told it would...

I'm soooo lost!!! :help:

 

[cj97tahoe]

maybe try another pcm. sometimes they just don't give a code. to go from getting 390mpg to 200; there must surely be a problem. my 97' with 4.10 gears, no cats, no egr, and a BB tuned pcm gets 349 hwy. But the tuned pcm gives fewer codes so I sometimes have to replace it with my stock pcm just while troubleshooting problems.

 

[cj97tahoe]

pcm from a junkyard or even a friend with a 98' or 99' is worth a try

 

[98_2DR]

I took the Tahoe to a second shop who said they would give a second opinion on the first shops verdict. They had truck all day testing everything they could, no fuel leaks, pressure tested good and held pressure, they cut observation holes in front and behind cats stuck in lights and a scope and said the cats look new. Ran some other tests I can't remember all of them but then called and said I have a manifold gasket leak on drivers side and that issue will cause a rough idle and poor fuel economy. Is this true? I know this is a common issue with these trucks and with 201,000 miles it seems legit, but at $500 parts and labor that's a big gamble. What do you guys think?

 

[lesterl]

If you have never had LIM gaskets done, then it is probably time.

That being said the one plug being wet sounds like a missfire or overfueling on that cyinder (could be weak compression also).

What are the fuel pressue numbers Key On Engine Off? Key On Engine Idling? Revved a bit to like 1500 RPM? How long does it hold pressure when turned off?

What are the O2 sensors reading? Specifically bank 1.

Sounds like a sticking poppet or something....

You did check the cap/ rotor/ wires/ etc?

 

[98_2DR]

The shop didn't give me that info. I did check and recheck all the plugs wires and cap/rotor the old plug was wet the new ones I installed are dry as a bone all the way around still. As far as the misfire both shops explained they checked for that at idle, under load and throughout driving and not one misfire registered.

A stuck popet or fuel injector, if it were this would a good fuel system cleaning fix this?

 

[samotahoe]

ask them what the vacuum readings were. average should be 18-21"hg. if its lower than normal then you might have an issue with vacuum but higher could lead to valves or such. and that shop that told you cats wear out? and to put straight pipe?... they are a joke, I would stay clear from that place. I work at a shop here in cali, and we do smogs here also. if I told someone to do that I could lose my job lol

 

[98_2DR]

Went today and had the dumped exhaust fixed with a Y pipe and stock looking tail pipe in stock location, still sounds nice but not dumped anymore, I have to say what a difference, however I did drive like 100 miles today and noticed my temp gauge not getting above 100deg, and after talking to the chevy dealer they did some checking, not sure what since I didn't watch them do it and they didn't charge me, they directed me to local auto parts house or the parts counter there and said my thermostat isn't functioning properly even tho it's registering 195, it's not opening or closing on time, and my temp sensor that talks to computer was flat out dead. Went to autozone and $30 later I have me the two new parts to install and they said if that if that registers +/- 40 degrees it will dump fuel into the mix so that could be part of the issue. I figured its about 30-45 min worth of work and it was cheap maintenance anyway ill go ahead and do it... All prayers would be great and ill post any info into the fix when I have some...

 

[samotahoe]

that does make sense but I don't think it would make that much of a problem. let us know how it turns out

 

[lesterl]

If the sensor was open, I am SUPRISED it started at all, if it thinks the temps are at open resistance (-40 +/-), then it DOES dump TONS of fuel (same thing a choke would do.) Betcha got it this time...... Good luck!!!!