Help Request: Rough Idle, low mpg

[tahoesteve666]

Getting some vibration, very slight shudder at idle on a recently acquired 99 LT, unknown history other than mnt records, which show it was getting hot a few years back, getting 11 MPG in city.

So far:

a) I probed plug wires, didnt quantify drop, but seems each plug responded,
b) was able to get one of the ELM devices to feedback misfires counts, but they were zero across the board,
c) I dont see any external coolant leaks, but I may be losing coolant, maybe a pint over full gas tank I topped off and will watch to confirm, radiator cap feeds to the overflow reservoir and no leaks in the tubing
d) sprayed some carb cleaner around the hoses, no effect
e) new PCV and hose - no effect
f) no ECL & no codes. MIL not ready for evap (only have put about 300 miles on it, MIL not ready from day 1).
g) has new fuel filter, and new coolant with pressure test of radiator
h) did not see any signs of fluid mixing during visual inspection, internal levels look fine

anyone have suggestions?

 

[east302]

Try cleaning the throttle body and the idle air control valve.

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[tahoesteve666]

I have a new IAC on order as backup, seems like alot easier than all the cleaning. But will tackle cleaning the throttle body and current IAC this weekend...

I also posted summary of the historic mnt records:
Miles Date Maintenance
100959 Radiator Replace
103622 Oil, Alignment, Right Side Upper Control Arm
107181 Wiper Blade
110047 Oil
111631 Power Steering Hose, Pump, Oil
111634 A/C Service
114123 Battery Cable Tightened
114859 Oil, Brake Service Front/Rear
117336 P0122 TPS Sensor Replace
121962 Serp belt Tensioner, Idler Pulley Replace
122871 Oil
123706 A/C Manifold Hose, 44F, Oil
123712 Serp belt Tensioner, Idler Pulley Replace
125523 Oil
129141 Oil 5w30 Napa 21036
131644 Oil
134452 Oil, Air, Napa 26440
134814 Output shaft Bearing/seal, Transfer Case, Brake Service Front, Napa AD-7259B Pads
136089 Transfer Case Rebuild
136442 Serp Belt
137665 Oil
137880 Squeeky Belt, Belt Change
138389 Adjusted A/C Pulley
138981 Belt Noise/Compressor, Replace Compressor, Receiver Dryer, Accumulator, Dryer Tube
139401 A/C Compressor Replace, Carbon Clean Fuel Injector, Differential Service
141335 Belt tensioner, Serp Belt Gatorback GC04060967
143364 Oil, Heater Hoses, Antifreeze
143668 Blower Motor Resistor
144667 06/10/2015 Intake Manifold, Antifreeze, Oil
144680 07/30/2015 Wiper Blades
145692 08/05/2015 Oil
149119 10/28/2016 Oil
149145 02/13/2017 Oil Cooler Line Gasket, Oil Cooler Adapter

 

[east302]

Is the belt still squealing? They sure went through a lot of them after the power steering pump was replaced. If it’s still loud, check the pulley on the pump, the shaft should be flush with the pulley flange.

One thing to check would be a vacuum leak at the upper manifold and injector assembly interface. I see that they did the lower intake gaskets...wonder if they pulled the upper intake off as well for some reason. Anyways, here’s a link...

https://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/rough-idle-after-installing-spider-injector-1


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[tahoesteve666]

Belt not squeeling... yea, in the mnt notes, basically they finally addressed some core alignment issue and replaced the belt with a Gatorback (under warranty) and I guess the issue went away. Before my time obviously..

I will check fuel pressure stability tomorrow...

As noted previously, I had sprayed carb cleaning around the hoses and intake, no response, but I will try it again tomorrow as well..

I dont recall them changing out the spider..I did that once before (see my other thread on the previous Tahoe.....ugghh).

Im really suspicious about the coolant drop aspect...Im tempted to put some stop leak additives into both the oil side and the coolant side, but havent yet...rather go through the debug in more detail first..

thanks for link to the article, ive seen that article before, used it on the other Tahoe actually, but good to have a quick link to it..

 

[east302]

I missed the part on the coolant level dropping. Have you done a pressure test on it?


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[tahoesteve666]

yea, did a pressure test on it already. There was a rotted Y-connector in the coolant lines giving an external leak at the time, so we swapped that out, flushed and refilled and did a pressure test at that time, looked fine.... gearing up now to go do some followup testing

 

[tahoesteve666]

One other comment, maybe someone can clarify, it was suggested to clean the IAC..does the IAC have any influence on performance away from idle?? Since the MPG is low, about 25%, (11 MPG actual vs 14 MPG expected) it would seem roughness at idle is really just a symptom, but not a limitation to the current performance unless the IAC is also controlling during typical driving. I have a new one on order also, $6.50, but suspect its not the issue.

Also, the ELM scan tool indicated no misfires. Similarly, there were no codes and CEL, but correct me if I am wrong, it only makes sense that there are no codes related to this, and no CEL because the computer is not registering misfires which is why the scan tool is indicating 0 misfires in the first place, as it is just mimicking what the computer is indicating. Does this REALLY mean there is no misfire?? Is it possible to have misfires to the extent of a 25% hit in MPG and the computer simply misses it?? Seems doubtful to me. Even my previous Tahoe which had an injector issue threw random misfire codes. So what this seems to be telling me is this is not a Fuel or Spark issue. Basically, 25% seems significant and I need to associate that with something that can actually have a 25% influence.

I'm more inclined to focus on the a) coolant level aspect - basically gasket/seal issue b) vacuum leak and the Evap not ready, c) core engine - compression

 

[east302]

Yes, it can detect a misfire but not throw a code for that cylinder until a list of requirements are met.

What size tires are you running vs what is on the door jamb label? 11-mpg isn’t bad around town, but wanted to see if you had non-stock tire size and had corrected the odometer reading for it when calculating fuel economy.

I’ve had three of these (all 98s with the 350) and they all had a little quiver at idle sometimes. Nothing bad, just something that I’ve always noticed. What is the scanner showing for the rpm when this rough idle presents?


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[tahoesteve666]

so a followup question on the misfires..
Is it possible for the scanner to indicate 0 misfires across all cylinders but not be an exactly accurate representation of realiy? in other words, ..can i strictly believe the scanner...is it absolutely definitive? if it is 100% reliable, then doesnt this mean I can eliminate spark and fuel out of any potential issue (real or imagined)?

I am definitely babying the local driving right now...so 11 MPG would be best case (and in the interest of full disclosure, this is a suburban, not a tahoe)

With regard to tires.. 245/75R16, same as door jamb

Scanner showing about 625-640 rpm at idle

 

[tahoesteve666]

A little bit more data:
a) checked fuel rail. Pressure charged is 53-53.5 psi, after 5 minutes off 52 psi, after 10 minutes off 51 psi. Stopped after 10 minutes.
b) sprayed carb cleaner all over the place again on cold engine...nothing
c) pulled the radiator cap and watched response through fill port as it heated up (would only heat up to about 150 F (via dash gauge) from cold engine, idling. Fluid expansion brought the overflow reservoir from cold level to close to hot level (bit shy of it). Saw some bubbles escape to surface of the fill port, maybe a bubble every 10-15 seconds, but it was not continuous. I also used a turkey baster and pulled about 30-50 ml out and stored it in a clear water bottle. It looked very clean. I will let it cool and see if I can see any surface film but I suspect not. There was a very very very slight surface film when looking at the fill port, but I suspect that was just from internal contamination of the coolant path rather than any oil mixing.
d) I also get your point about the RPM at idle, you are checking to see if I am starving the engine correct? I checked a few other posts..seems idle should be a bit higher...and thus the comment about the IAC above?? Do you know best target idle rpm?? and is my logic/interpretation correct here?

thanks

 

[east302]

Here is the spec on the idle, GM gives the value with the transmission in drive.

What is the scanner showing for coolant temperature (as reported by the ECT sensor)? I’m not sure what thermostat you have, but would expect it to report around 190-ish when warmed up.




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[east302]

so a followup question on the misfires..
Is it possible for the scanner to indicate 0 misfires across all cylinders but not be an exactly accurate representation of realiy? in other words, ..can i strictly believe the scanner...is it absolutely definitive? if it is 100% reliable, then doesnt this mean I can eliminate spark and fuel out of any potential issue (real or imagined)?

The VCM is reporting the misfires based on the CKP and CMP sensor readings, so I’d say that if it’s not reporting a misfire then it’s not seeing a misfire.

Your idle speed looks comparable to the scanner reading that I saw on my 98 the other day.


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[tahoesteve666]

From the previous work and data I collected using the ELM, the warmed engine was holding steady at 194 F, not sure which PID this was offhand, would have to rerun to confirm. (While the radiator was just flushed, we didnt take tremendous care on the mix ratio, we just approximated since we thought there was probably leftover water in the system from the final flush rinse.) Seems this is thermostat controlled anyway.

I will check the in-gear idle RPMs tomorrow..but it sounds like Im not far off, if at all, from where it should be.

 

[tahoesteve666]

Update on the data:

a) So in a previous comment I referenced 150F on the dash... basically, this was the first small 'bar' above the larger mid 'bar' on the gauge, but I checked it against the ELM and the elm was indicating 88-89 C (194F) at this point. So basically, the dash gauge seems pretty much worthless other than as a reference to nominal.

b) I performed some scanning under 3 conditions: 1) engine off/battery on (obviously)/cold, 2) engine on/cold, 3) engine on (warm). I would say that getting info under '1' is suspect as many of the parameters seem to have stored values from history and dont reflect anything about current state, especially since the engine is not even on. Data under '2' is tricky also as all sorts of transients are occurring and represents open loop initially. Data under '3' seemed most reliable and stable', more accurately represents actual driving and is closed loop.

c) For the RPMs, I used PID 221191 (TCC Slip Speed), 221192 (Desired Idle) and 0c (RPM). They were consistent and with warm engine and parked , as indicated previously, it was 625 +/- for all and in Drive (Brake on obviously) it dropped t0 550-560 rpm for all. Quiver present under all conditions tested.

So summarizing my data interpretation so far: Idle RPM fine, Fuel Pressure fine, Tire Size correct, No apparent misfires via scanner, Engine Temp fine, no external leaks, carb clean spray does nothing, no codes/no CEL, coolant fluid quality looks ok, fluid levels look ok. And my quick spark plug probe seem to show all plugs plug reacting but MIL Evap not ready still exists

Interpretation: Probably dont need IAC replace for one. Think injectors, compression/leak and internal vacuum leak as only possibilities

Plan: Will still clean throttle body, probably will replace IAC when new arrives just to test and then replace original and keep new as spare. Will pull sample of hot engine oil and do visual analysis once cool. Then reevaluate again (Im trying to push off compression and injector work as much as possible as I cant see or move well anymore)

 

[tahoesteve666]

Heres an update: So I check fluid levels and it seems Im losing coolant..quite alot actually, but I see no external leaks and the previous pressure test was good. I topped off the fluids again and going to monitor it to confirm. For the hell of it, I baselined the vibration magnitudes using a tablet with a a downloaded app and measured in 3 physical locations inside the cab, just to get the feel of it. Consistent signal for a given location, but location to location had significant variation.

a) Waiting for a block tester and test fluid to show up just to check to see if any combustion gas is making it into the coolant path.

b) Also waiting for some coolant dye to confirm lack of external leaks, if any signal in the exhaust and will also check the plugs after doing a radiator pressure test with the fuel line disabled, as well as without plugs again white paper and see if anything shows there.

c) planning on compression test as well

d) will pull radiator cap again and see if any bubbles

e) will pull a sample of oil and check on a hot plate for bubbles vs smoking

f) will repeat the radiator pressure test as well once that tool comes in (previous one was not mine) and really make sure whether its holding...or more likely..determine when its holding and when it isnt!!! Going to to try running this hot after an open cap warmup.

not sure what else to do for now, but really want to make sure about any core leak issues..will post results once tools come in.. Any thoughts/suggestions..please chime in..thks

 

[tahoesteve666]

Bit more info: Engine warmed up, cap off radiator, 87 C on the coolant temp, heater on high and blowing hot, I think it was 157 F (I may have spaced and it might have been 147 F, but I tend to think 157 F) off the middle console vent. Absolutely no oil vapor on the inside of the window shield with defroster cranked, nor did I see any drips in the cabin under the dash.

a) block tester showed up, absolutely no color change on the test fluid, so thats really good news.

b) dye has not shown up yet but I did take a closer look at the exhaust, not a trace of any white smoke. There were moisture drops. I put a nice white sock to collect it, smelled it, seemed like just water to me from the CxHy + O2 -> CO2 and H2O type reaction..but I will check with dye once it arrives.

d) really no signs of any bubbles, which I think is supported by both the test fluid test and the fact that after multiple pumps on the tester bulb, I was generating some level of vacuum as the tester bulb would not fill up and simply held collapsed state until I broke the gasket seal and allowed air to enter.

 

[tahoesteve666]

Bit more info. Did a oil smoke test on material captured from the dipstick using a pump, shortly after running hot.

There were small bubbles evolved, but nothing violent. I suspect the small bubbles are simply trapped gas inside the oil from all the turbulence of running the engine, rather than incorporated coolant from a leak. I think a better test would have been to pull vacuum on the extracted oil in a clear container, prior to the smoke test, but Im not set up for that. I will have to rig something up for that...but my current interpretation is no coolant in the oil, but it is not conclusive.

 

[tahoesteve666]

So here is the latest on this issue:
a) ran dry compression test: Driver Side: 155, 172, 130, 162 - Pass Side: 180, 155, 180, 185. so I would say there is one cylinder (130) with a definitely issue. Tried wet on this cylinder also and it improved only to 135.
b) did end up changing cap, rotor, wires and plugs also
c) added mechanic in a bottle...Bars HG-1 and followed directions to the letter. did seem to help the extent of moisture in the exhaust, will have to watch it to confirm though...as the weather is changing alot and want to be sure no contribution from that.
d) also had added dye to the coolant (prior to HG1) and checked exhaust, but did not really see anything, nor did I see anything obvious on the old plugs when they were later removed and inspected.
e) latest tank of gas was only 9 mpg....lower than he 11 mpg before

so basically, Im still questioning coolant loss, low mpg, vibration (at idle only, I dont really notice it otherwise). And since the HG1 is recent, i topped off the coolant, refilled the gas tank and will put a tanks worth of mileage on it and reevaluate things (coolant loss, mpg and idle vibration primarily) and post then...

 

[tahoesteve666]

Followup ..probably final for this issue.
MPG after a tank or two of fuel is now right at 13mpg in the city (easy driving).
Vibration still there, but I dont seem to be losing much coolant now.

I do notice the thermostat seems to open up initially a bit later than normally. Dash temp gauge peaks to about 210. Then, when it first open, it cycle low to about 150, but after that it is pretty stable at 190.

I fully believe vibration is a result of the compression imbalance (cyl 3 low). Will update if it craps out, but I suspect I'm stable for awhile.