I have a ’95 Suburban with a 5.7L V8, 4-spd Auto and 4WD. And 230,000+ miles. I bought the truck in 2000 with 86,000 miles and have kept very good records over the years. In the past year the mileage has declined from an average of 14-15 MPG down to 8-9 MPG. I used to never see numbers like that unless I was towing snowmbiles to Vermont.
When the mileage first started dropping I found a crack in one exhaust manifold. I replaced it and the mileage picked up for a tank or two, but still declined.
Since then I’ve replaced the other exhaust manifold, the O2 sensor, rebuilt the TBI, replaced the cap/rotor/wires and spark plugs, and replaced the PCV.
The truck starts and idles fine. The check-engine light is not on. It seems to have the same acceleration getting on the highway (I had a plugged cat so I know what that feels like — I’ve replaced the entire exhaust with a stock setup from GM). The spark-plugs have a gray-tan color with some deposits, nothing out of the ordinary from my experience. There’s no smell of gas like from a leak (I replaced the gas tank two years ago when it rusted out).
There doesn’t appear to be a vacuum leak — I get 20″ at idle, no hissing sounds, hoses aren’t cracked or checked. I haven’t tried spraying the intake seams with WD-40 to check for gasket leaks, yet.
There are two definite symptoms. One is the horrid mileage. The other is when I hold the throttle steady just above idle the engine surges a few hundred RPM up and down.
Anyone else run across this before? Any ideas on what else I can check? I’m looking for diagnostic tips, as I don’t want to waste any more money throwing parts at the problem. Thanks for any and all help.
kostel
Norwalk, CT
103 příspěvků · Připojeno 2008
špatná spotřeba plynu
This may sound dumb but have you checked your tire pressure? I have 10 ply tires and visually they look fine but found they can be down nearly 25 psi and didn’t notice the differance in driving because it was lost so gradual but boy I sure could feel a differance when I filled them. Also the air filter is often overlooked but very important to the smooth flow of fuel/air mixture.
27 příspěvků · Připojeno 2009
have you checked or replaced your fuel filter and did you check fuel pressure 9to12lbs as far as the surge unplug your egr valve and see if this symtom goes away
3441 příspěvků · Připojeno 2008
When you are at cruising speed or a standard residential speed are your RPMs higher then normal, when you are in town do you drive in overdrive or drive/3rd gear? Another dumb question, but your story is odd, is your 4wd a stick or push button setup? Your engine temp should be sitting in the 180 degree, if your t-stat is shot this will effect your mileage as well.
99 K1500 Suburban LT „BEAST“
Hypertech III, K&N, true dual
285/75/16
___________________________
Jason
KirkW Discussion starter
94 příspěvků · Připojeno 2007
Thanks for the tips, all good ones.
I just changed the air-filter, and I change it every 15,000 miles.
The fuel-filter has 20k miles on it now (I change it every 30k). I have not checked the fuel-pressure, but the fuel-tank and everything inside (sender, pump, strainer) were replaced 20k miles ago. I rebuilt the TBI 10k miles ago (which included the fuel-pressure regulator) so the pressure by be correct. But I’ll put ‘fuel pressure’ on my list of things to check.
I got four new tires 12k miles ago. I haven’t checked the tire pressure recently, so I’ll put that on my list, too.
The transmission shifts correctly, it goes into over-drive and I can feel the torque-converter lock-up on the highway. The tach shows the correct RPMs for the speeds I’m driving. I have the shift-lever 4WD. I replaced the infamous thermal-switch for the front-axle with a Warn vacuum-switch — now it shifts in and out of 4WD very quickly. Judging by the «4WD» light on the console and the way the truck feels when going around a tight corner, I don’t believe the front axle is engaged when it shouldn’t be.
I can’t recall when I changed the thermostat but it’s a stock 195° unit. According to the temp gauge on the dash (and the output from the heaters), the engine seems to be coming up to temperature. However, I’ll check the ECT sensor for the computer to make sure it’s reading the right temperature.
I have the factory service manuals and a cable to connect the ALDL to a laptop. However, I don’t think I have a good program to emulate a «Tech 1» tool so I can read the short- and long-term fuel trims and other stats. Any recommendations?
At this point I’m going to RTFM, and follow the GM steps one by one (assuming I can get my scan-cable to work). Thanks for the tips — I’ll check everyone up to speed on what I find!
kostel
Norwalk, CT
99 příspěvků · Připojeno 2009
Couple more things.
Any chance brakes are dragging?
Ever tried seafoam? Stuff is amazing.
Are you using another gas station on a regular basis? (some have poor quality gas)
1665 příspěvků · Připojeno 2006
I didn’t notice if you had replaced or checked the MAI- which I think is the air mass sensor usually found between the airbox and the intake snorkle?
What about a compression check?
My 1998 5.7 Suburban 2wd has 208,000 miles on it now,and it manages 20+mpg on the interstate at 65 mph.It gets about 14 mpg in the city with careful driving.Your mpg is waaaaay short of what it should be even with normal wear at 220,000 miles.
1998 suburban-
1/2 tun
199500 míle
Řeka
Ridge,LA
KirkW Discussion starter
94 příspěvků · Připojeno 2007
I did a compression check 10k miles ago when the fuel mileage first starting dropping. Here are the results:
Cylinder / Dry / Wet
1 / 150 / 174
2 / 157 / 176
3 / 150 / 175
4 / 154 / 175
5 / 147 / 175
6 / 149 / 170
7 / 143 / 171
8 / 160 / 180
The numbers are well within factory specs; nothing jumps out as significant. The oil consumption is low — barely a quart in 4k miles. And since I installed new valve-stem seals I have no blue exhaust smoke at startup. So even though the wet compression test is higher, it’s still normal and not a reason to believe the rings are worn.
This truck doesn’t have a MAF in the intake snorkel. Instead it senses intake vacuum via a MAP sensor. The numbers from the test scanner match the vacuum gauge. This truck doesn’t have an air-temp sensor, but the ECT (engine-coolant temp) is showing the correct values.
The scanner also shows the surge coinciding with the EGR. When I hold the engine just above idle (1100RPM), the computer turns on the EGR, the RPMs drop a lot, the computer turns off the EGR, the RPMs recover, the computer turns on the EGR. and so on. It’s a «Negative Back-Pressure EGR Valve» that balances the amount of EGR with the pressure in the exhaust manifold. Running some tests from the factory manual shows it’s working correctly. So I believe the surging is not caused by the EGR, but rather a problem elsewhere in the engine that is exacerbated by the EGR.
Disconnecting the EGR is not really an option. The computer periodically tests the EGR valve by turning it on and off during steady cruise and noting the O2 sensor readings. If the readings don’t change, then it assumes the EGR is broken and sets a trouble code. I’d rather not drive around with the «Service Engine Soon» light staring me in the face.
Once I figure out where to get the fittings to test the fuel pressure I’ll be doing that.
Right now it’s warmed up to a balmy 20°F outside (since Suburbans are too big to fit in a typical suburban garage — oh, the irony — I have to do all my work outside). So I’m going to bundle up to go outside and run some more tests, but frankly I’m getting a little fed up at the moment. I may just go to the airport and fly my plane instead (no computer, no vacuum tubes, just a simple carbureted four-cylinder engine. ).
Thanks again for the suggestions — further bulletins as events warrant.
