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Thread: 6.2 in an RV Low on Power

  1. #1
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    Default 6.2 in an RV Low on Power

    I've been fettling with this RV for a few years now as a long term project. Took a while but I've finally chased down and removed/replaced all of the broken bolts in the accessory brackets (5 total between the alternator and power steering pump) and the engine starts & revs fine. I have been running it around in my neighborhood the past couple of weekends now that I have the belts all aligned and tightened properly but the problem is, it won't accelerate over 35mph. I'm not trying to push it very hard but it just won't go any faster. Engine continues to rev, doesn't have any hanging idles or flat spots. Transmission fluid is still clean and red and is above the Fill line. Shifts are rough but they were rough in my other 6.2l RV and that got over 60mph.
    Couple cromulent facts;
    -33,000 miles on the engine
    -Its got a Stanadyne fuel filter assembly up on top of the intake which is a hell of an upgrade from my original RV with the OEM setup
    -Currently doesn't have an air filter, housing is there I just haven't bothered to put one in since I'm still troubleshooting the long list of electrical gremlins and not going anywhere in it yet
    -Has a momentary switch for the GPs, sometimes it works sometimes it doesn't but either way the engine starts within seconds of cranking
    -Exhaust is clear, no smoke whatsoever
    -While working on the accessories I removed the fan and fan clutch and on the bench the fan spins ok but it feels a little "gummy". I can give it a twirl and it will spin around once but no further
    -By the time I'm up to 30mph the trans has already shifted to 3rd. I seem to recall on my earlier RV the shift points were 15-18mph into 2nd then around 40mph into 3rd so this seems like its shifting way early

    I am going to bring an infrared thermometer with me next time I drive it because while it brakes OK I want to check on the hubs and maybe I've got a brake that is dragging. I have Vol's I & II of the TDP maintenance & troubleshooting books but irritatingly I cannot find them.

    Thanks for the help.

  2. #2
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    What chassis, model year, weight class, gear ratio, and tire/wheel size?

    (In no specific order)

    Verify your fuel supply to the IP (get a pressure gage on it, between the fuel filter and IP). It should be about 4 PSI at idle, and not fall below zero, under any condition or load. Low or restricted fuel supply will not only reduce power, but also causes early, harsh auto trans shifts. It should produce at least a little black/gray smoke while throttling. If it doesn't, it isn't getting enough fuel, or the timing is retarded. Too retarded produces white smoke. Verify free fuel flow of the return to the tank, and the tank is properly vented ("Diesel Only" fuel cap, or other venting system). A restricted return can retard timing significantly. Ignition rattle should be sharp, not dull. Advance the timing so the rattle is at the early edge of sharp. Too advanced will increase EGT.

    Verify fuel quality (not contaminated with something else, like water, gasoline, or a bio, pull a sample at the filter water drain into a glass jar and let it sit for an hour), and that it has no leaks ahead of the lift pump (small leaks after the pump make a mess, but don't usually effect running). Any air in the system reduces fuel volume and retards timing. The label on the fuel filter means nothing if it isn't allowing good fuel flow, for whatever reason.

    Verify the vacuum pump is producing at least 20" hg, as well as a properly routed throttle vacuum valve, and working trans vacuum modulator. I don't think this is your immediate problem. If it's able to accelerate to 35 MPH, it should be able to accelerate further in 3rd, unless power is decreased, for whatever reason.

    The fan clutch sounds normal. Make sure it's a HD model with the bi-metal coil on the front. If it isn't, it may not be able to keep up under heavy use. Either way, not your immediate problem.

    To check for dragging brakes, jack the wheels and give them a spin by hand after pumping the brakes hard a few times. You should be able to feel any significant drag. If you can spin them by hand, there's not enough drag to slow it down under power. If it has a shaft brake (hand brake), make sure it's working and releasing.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  3. #3
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    1984 G30 chassis with dually rear end, not sure about the gearing

    According to the info decal inside the GVW is in the 11,000lb range
    Tires are 8.75 x 16.5
    When I say the exhaust is not smoking, it’s not smoking a cloud but upon throttle up there is a faint white cloud.
    Lord knows the age of the fuel that’s left in the tank, I’ve had it in my driveway for 3 years, and bought it from a former member here who told me he hadn’t run the RV in 5-6 years prior to selling it.
    Fuel cap doesn’t indicate the Diesel Only

    Last edited by Bigshankhank; 09-06-2021 at 15:59. Reason: Photo links

  4. #4
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    That leaves a lot of variables. White smoke indicates cold or choked combustion. Since your intake is wide open, that leaves cold (too little heat for complete combustion). Poor fuel quality, restricted fuel supply, and/or retarded timing can cause that. I would start with flushing the fuel supply with fresh fuel, and make sure fresh, uncontaminated fuel is supplied. Change the fuel filter once you're pumping clean fuel. If you find black or very dark goo or chunks in the fuel (algae), add a biological cleaning and treatment. You'll need several fuel filter changes if the tank isn't cleaned and lines thoroughly flushed. Leave the fuel cap slightly loose until you can replace it with a properly vented, Diesel cap.

    If you would like to upload photos, go to Community/Albums at the top of the page and create a photo album. You can link them into posts anywhere in the forums.
    1985 Blazer 6.2
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    dmaxmaverick@thedieselpage.com

  5. #5
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    Grrr apparently I set my photos to auto delete on the hosting site, I’ll fix them and repost.

  6. #6
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    Quote Originally Posted by DmaxMaverick View Post
    Poor fuel quality, restricted fuel supply, and/or retarded timing can cause that...
    Or bad spray pattern...
    The Constitution needs to be re-read, not re-written!

    If you can't handle Dr. Seuss, how will you handle real life?

    Current oil burners: MB GLK250 BlueTEC
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    Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510

  7. #7
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    Quote Originally Posted by JohnC View Post
    Or bad spray pattern...
    Certainly possible, but the first indication is usually difficult starts. He says it starts and runs fine, just doesn't make power. It sounds more like what and/or when it's spraying (or not), rather than how.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  8. #8
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    Photos restored.

    I've been holding off putting new fuel in the tank because I want to drop it and get it flushed anyway, no sense doing that with more weight than necessary. Plus, the fuel gauge is pinned past Full so I want to clean and service the sending unit and hopefully find and eliminate the short in the circuit.

  9. #9
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    If you are planning on cleaning the tank, I suggest doing nothing else with the running issue until that's complete. If the fuel is contaminated, you really don't want to push it around the system. Also, after the fuel tank and fuel is clean, run the return line into another container until it flows that clean fuel. If you don't, you risk flushing the dirty system right back to the clean tank.

    If the fuel gage is off, and cleaning/replacing the sender doesn't help, the gage itself has a resistor on the back of it. When that fails, the gage goes wonky. The resistor is an easy replacement, though. I don't recall the resistor value (I'll bet google does).

    If you fixed your pics, you'll need to edit your original post with updated image links, or just post them again.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  10. #10
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    Quote Originally Posted by DmaxMaverick View Post
    If you are planning on cleaning the tank, I suggest doing nothing else with the running issue until that's complete. If the fuel is contaminated, you really don't want to push it around the system. Also, after the fuel tank and fuel is clean, run the return line into another container until it flows that clean fuel. If you don't, you risk flushing the dirty system right back to the clean tank.

    If the fuel gage is off, and cleaning/replacing the sender doesn't help, the gage itself has a resistor on the back of it. When that fails, the gage goes wonky. The resistor is an easy replacement, though. I don't recall the resistor value (I'll bet google does).

    If you fixed your pics, you'll need to edit your original post with updated image links, or just post them again.
    All good info, I have no real need to run it anymore now that I've sorted a couple of electrical drains and the broken bolts. I started trying to take the gauge cluster out a few weeks ago to see if the problem might be easier to get to but I couldn't get enough slack in the speedo cable so I shifted focus back to the engine once and for all.
    The images show up again in Post #3 on my screen, if they aren't working on your end I'll have to go back in and edit it if I still can.

  11. #11
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    They may be showing on your system, but this is what I see:

    Screenshot-20210907181711-350x832.jpg

    If the links don't work, try uploading your pics directly to the post using the image link at the top of the post editor. You can upload directly from your computer or phone.
    1985 Blazer 6.2
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    dmaxmaverick@thedieselpage.com

  12. #12
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    Contaminated fuel could account for you poor performance and the problems with the fuel gauge. I'd deal with that next.

    Ditto on the pictures.
    The Constitution needs to be re-read, not re-written!

    If you can't handle Dr. Seuss, how will you handle real life?

    Current oil burners: MB GLK250 BlueTEC
    New ride: MB GLS450 - most stately
    Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510

  13. #13
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    OK, gonna try and upload them again,
    Rear differential, no markings as to the gearing inside. Again, 1-ton chassis with the dually rear

    Fuel cap, no markings for Diesel Only

  14. #14
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    Since I'm not going to run it for a while until I can flush the tank, I have moved on to deciphering the non-functioning gauges on the dash. I realize that this is a forum specific to the 6.2l Diesel engine so if I need to post these questions elsewhere let me know and I'll continue to focus this thread on the power issue.
    On my dash, 3 gauges are pinned past the max; Fuel, Water Temp and Oil Pressure. I removed the cluster and pinged the circuit mat on the back of it, everything is making good connection. I wiped it down with some cleaner anyway and put it back together. I got lucky and found that the female spade terminal on the wire to the oil pressure switch was damaged so I replaced that terminal and now I have a working gauge again. I would like to put a mechanical switch & gauge in there but maybe another time. The Fuel gauge will likely be something back toward the tank sending unit so I'll address that when I drop the tank. But the water temp switch has me stuck. I popped the wire off the sensor in the back of the radiator and couldn't find a pin in the back of the plug behind the gauge cluster that made connection. Tracing the circuit mat pointed me toward the prongs that should be either making contact with the sensor or the ground. Should be the green wire (next to the open space) in this photo

    However with everything plugged in and the ignition switch in the On position, jumping this wire directly to the sensor did nothing. However jumping the sensor to the terminal on the back of the cluster highlighted in this photo

    brought the gauge to life. Except, that terminal doesn't connect to anything on the circuit mat so my thought is for some reason there is a wire separate from the main harness plug that has a ring terminal that is connected to this stud. The wire on the sensor at the radiator is black w/ green stripe and I can trace it all the way to the firewall, but from there I cannot find it under the dash. I've been going through wiring diagrams online because I cannot find the thumb-drive where I already downloaded all of this stuff to keep things simple, but I'm wondering if anyone here is familiar with these vans and the instrument panel wiring and can help me out.

  15. #15
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    The sender in the radiator is for the coolant level (Low Coolant Lamp). If you have a time when the coolant is low, for whatever reason, take it out and clean it. It can build up with crud and either not work, or indicate low all the time. The temp sender is in the head or coolant outlet (thermostat housing). I'm not much help without a diagram, as I only have them for the trucks. It's anyone's guess without one with the chassis upfitters.

    That filler cap is way overdue. Just replace it, they're cheap.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  16. #16
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    So yeah it dawned on me last night that that was the low coolant sensor so I feel pretty damn smart having spent hours trying to figure out why it wasn’t working on my temp gauge.
    Anyway, may not get a chance to wrench on this for the next few days but I’ll seek out the actual temp sensor and go from there. I actually have a wiring diagram for the RV house and I have one on a thumb drive somewhere for the chassis, but since so was barking up the wrong tree hopefully it’ll be more clear when I am linking the right parts together.
    Thank you for your help.

  17. #17
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    Did sneak down to the garage for a bit to continue sorting the temp sensor. Found the actual sensor in front of the driver's side exhaust manifold and sure enough it has a dark green wire same as the back of the gauge cluster plug. Snapped the terminal off the end of the sensor and it has continuity back to the green wire on the plug and doesn't indicate a ground. Tested the sensor prong (not the threaded body) itself and it is not grounded, either. I jumped a new wire from the sensor to the backside of the plug and it still pegs past max. So either I've got a bad gauge, or I am missing something obvious. Is there another sensor on the other head?

    Oh and a correction to my original post here, I have a Racor aftermarket fuel filter, not a Stanadyne.

  18. #18
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    It doesn't matter what sensor you're talking to, if the connector has continuity back to the gage. That's the one. The 1984 (truck/SUV) glow plug system has its own coolant temp switch and separate relay (usually installed at the rear of the head), unless it's been upgraded to the 85+ (integrated switch/relay box, installed on top of the engine, near the rear, driver side valve cover).

    It's possible the gage and/or sensor is bad. Pegged full hot is a closed switch, while open (or disconnected) is full cold, assuming the gage is working correctly. Somewhere on the interwebs is a resistance chart to verify what the sensor should be indicating. Read the resistance at a known temp, then change that temp (hot and cold water, for example), and read again. The value should change accordingly. If it doesn't change, or is reading a dead short or open (infinite), the sensor is bad. The fuse block should have a fuse for the instruments, so check that (doubtfully failed, with the readings you're seeing).
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  19. #19
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    The sluggish ...no power issue is likely due to a lack of fuel to the IP (volume)

    The transfer pump on these were a mechanical pump on the RH front of the engine.

    The fuel filter can also be a culprit.

    As has been mentioned....cleaning the system is a good thing.
    A clean fuel system, fresh fuel, fresh filter and if it were me I would stuff in a fresh fuel pump too....

    A failed transfer pump can result in minimal fuel to the engine and exactly what you are seeing.....

    At an idle there should be about 5 psi of pressure to the IP or at the filter inlet......

    With the engine idling ....open the bleeder port on the filter.....Fuel should squirt out....if there is no pressure and the engine dies, then the transfer pump (On the engine) is not working.....
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  20. #20
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    Terminology: a bit of clarification to (hopefully) prevent confusion and keep us on the same page.

    Fuel Lift Pump -the mechanical pump located at the lower/front/right side of the engine block, ahead of the starter (in this case). If the chassis has an electric fuel lift pump, there will be (should be) no mechanical lift pump. GM spec. is 4 PSI at idle, and greater than zero at any engine power level. This pump only moves fuel from the tank to the injection pump transfer pump. It is a simple, traditional, diaphragm-type pump, operated by a lobe on the cam, not unlike previous GM V-8 engines.

    Transfer Pump -The INTERNAL injection pump transfer pump, not independently replaceable or serviceable. This pump simply moves fuel from the IP inlet to the High Pressure distributor pump inside the IP. This pump is capable of drawing fuel from the tank (all else being healthy) however, engine power will be reduced in the absence of a positive-pressure fuel supply (such as a failed fuel lift pump or significant restriction) at the IP inlet.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

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