Workhorse Chevy Loses Power After Warming Up

[rvgeorge]

I am understanding you to say, the RV runs fine when cold or when you first start out then loses power. The engine control module, (ECM) has two modes, open loop and closed loop. Open loop uses predetermined values from the OEM to control the engine until it reaches an optimum operating temperature. It then goes into closed loop mode where all of the engine sensors provide the input to optimize the engine performance. My first test. I would disconnect the ECM temperature sensor at the engine to keep the RV in open loop mode. Test drive to see if the if the problem goes away. If it does not, it leads me to believe the problem something other than what ECM controls. Such as the catalytic converter, fuel delivery, ignition etc. If the problem goes away, this is NOT the resolution, it is just a test. My gut feeling is that one or more of your engine sensors to the (ECM) has a bad ground connection. The ECM is thrown into limp mode because it does not know what to do. Typically, the ECM grounds, are found on the frame in the engine bay. (Note!!!!!) do not test grounds with a digital multimeter. They do not produce enough load to adequately test the connections. Use something that creates load such as a light bulb. Good luck Please let us know what you find.

Good info thanks, I was just informed that it is more than likely the fuel pump, it is being changed and hopefully I'll know by tomorrow. I'll post either way, if its not I'll try that temp control thing. Repairs so far have been an 02 censor, complete tune up, fuel filter, 2 new Cat's, fuel relay, now fuel pump. Fuel pump makes sense to me now because AI told me that fuel issues will not show "check engine" GM left that out of their program on these units.

 

[Mark_K5LXP]

My bluetooth OBD interface is always connected in the RV. I have an old phone (no service) mounted to the dash running free torque as a mini gauge cluster as I go. I have a tablet running torque pro I can do more gauges and deeper diagnostics plus logging. Without this data all you can do is guess. With old rigs like ours, good luck finding anyone that will take the time to solve a problem at any price. The more self reliant you are even just gathering symptoms and data for someone else the better your chances of resolving issues. For the price of a cheap interface and a free app there's no reason not to try.

Mark B.
Albuquerque, NM

 

[John From Detroit]

My bluetooth OBD interface is always connected in the RV. I have an old phone (no service) mounted to the dash running free torque as a mini gauge cluster as I go. I have a tablet running torque pro I can do more gauges and deeper diagnostics plus logging. Without this data all you can do is guess. With old rigs like ours, good luck finding anyone that will take the time to solve a problem at any price. The more self reliant you are even just gathering symptoms and data for someone else the better your chances of resolving issues. For the price of a cheap interface and a free app there's no reason not to try.

Mark B.
Albuquerque, NM

Well Mark. We hams do tend to do technical stuff WA8YXM since 1968. Extra class today though

I've used a couple of programs And a couple different adapters..
Right now the adapter is a FIXD (but do not subscribe to their over priced service) and TORQUE PRO
My choce of a 'droid phone or tablet.

I like being able to monitor more stuff.

Plus last repair trip I showed the phone (log) to the counter person he called a tech to clear it.
Torque should have cleared it. problem has not returned (This one locked the blang light on)
(Engine did not warm up fast enough) To 7734 it did not.. Was at operating temp before I made it out the parking lot)

 

[rvgeorge]

thank you, I will look into getting one.

which obd adaptor should I get, one of the ones I looked at said android only, I have an iphone, not sure but I dont think it wont work. I

 

[uchu]

which obd adaptor should I get, one of the ones I looked at said android only, I have an iphone, not sure but I dont think it wont work. I

This one is supposed to work with iPhone:

https://www.amazon.ca/vLinker-Bluetooth-Diagnostic-Android-Windows/dp/B088LW211V/ref=pd_lpo_d_sccl_2/132-7486499-3100425

 

[John From Detroit]

which obd adaptor should I get, one of the ones I looked at said android only, I have an iphone, not sure but I dont think it wont work. I

I am not sure it matters on the adapter (But not sure means it might)>
What matters is the software.
The adapter just turns the ECC data stream to a BLUE tooth Stream.

But if the program to read it won't run on an incomplete ope;rating system device...

Well I have androids.

 

[Ex-Calif]

My bluetooth OBD interface is always connected in the RV. I have an old phone (no service) mounted to the dash running free torque as a mini gauge cluster as I go. I have a tablet running torque pro I can do more gauges and deeper diagnostics plus logging. Without this data all you can do is guess. With old rigs like ours, good luck finding anyone that will take the time to solve a problem at any price. The more self reliant you are even just gathering symptoms and data for someone else the better your chances of resolving issues. For the price of a cheap interface and a free app there's no reason not to try.

Mark B.
Albuquerque, NM

My buddy Rob thinks I am too techie...

But you are another level. I bow to your techiness

 

[rvgeorge]

This one is supposed to work with iPhone:

https://www.amazon.ca/vLinker-Bluetooth-Diagnostic-Android-Windows/dp/B088LW211V/ref=pd_lpo_d_sccl_2/132-7486499-3100425

thanks.

 

[DonTom]

I would guess a flakey CPS.

I have seen three bad CPSes (Crankshatift Position Sensor) units work great when cold and crap out or get very flakey (and very hard or impossible to restart when warm). Fine when cold.

Happened in my Y2K RV (while in Canada) as well as my Jeep and in my Dodge Ram truck. I got no codes in either of the three at first. I got code for a CPS error just after my Y2K RV was towed to a shop in Canada. And then it started. But that was fine, because then it showed the problem. Same with the Jeep, see below. You get the CPS error MIL code SOMETIMES just after all is running normally again.

I mentioned the Jeep here a few years ago.

They should be changed at 100K miles regardless, working or not. Also a good idea for the fuel pump, so that was not a waste, even if that was not the problem.

But my bet is on the CPS. Based only on my own vehicles, three of them.

All my CPSes failed just after 100K miles. My fuel pumps usually last a little longer than my CPSes.

-Don- 84701

 

[John From Detroit]

I have seen three bad CPSes (Crankshatift Position Sensor) units work great when cold and crap out or get very flakey (and very hard or impossible to restart when warm). Fine when cold.

When we put a new (used) engine in my class A it would not start.
Mechanic ask me about one dash indicator.. (Tac) did not flicker

Well the replacement engine had one difference we felt may have been signifigant,
That sensor.. Some oil seals were arranged differently.. So we used the old one.

Tarnished connections.. Unplug and replugged a couple of times and VAROOM.

Reason for post::::: That can indeed do it.

As I said. sometimes electronics can be affected by heat.

 

[rvgeorge]

Its is not the fuel pump, I picked it up yesterday afternoon, brand new Workhorse GM fuel pump, they insisted on putting in the right one so they searched, found one in the USA somewhere and shipped it to the shop in Canada. No idea on the cost yet, we have an account and they said they'll send it later. I drove it 50 KM (about 31 miles) It ran great, I was happy so I started to drive home to tell my wife the good news. Its been an awful summer not being able to really go anywhere and when try we sit on the side of the road waiting for it to cool down so we can go again. Anyway just as I start heading home first stop sign its still running fine, go to pull a way from the stop sign and no power, keep my foot on the pedal and very slowly it starts to move, cant get anywhere near the speed limit of 60KM Hr (40 mph) limped it about 2 KM with flashers going to get her home, I'm sure there were a few pissed off people behind me as it was a 2 lane road. Oh and once again no check engine light came on so no codes. If I haven't mentioned it yet it only has about 55,000 miles on it and it is very well maintained both mechanically and appearance most people are surprised its a 2008.
Back to the shop in a couple of days, summer is almost over, not sure if we will even get to use it this year.

 

[Mark_K5LXP]

When it's that pronounced of a problem, observing an OBD reader while underway will indicate at a minimum what the issue surely isn't, and possibly what it conclusively is. Either you can do the diagnostics or leave it in someone else's hands to do so, but it's going to need to be done. The try and see method of repair can work sometimes but unless you want to drag this out and possibly spend thousands on parts your RV doesn't need, you or someone you trust will need to do some actual troubleshooting.

Mark B.
Albuquerque, NM

 

[rvgeorge]

I just bought one of those OBD readers and I downloaded the Torque app onto my phone so I will be trying it out next week on the way to the shop. The shop I use is very trustworthy, they fix all my cars. Problem is you go there and its too short of a distance so it doesn't act up on the way, most time you park it leave the keys so when they hook up there is nothing wrong. Next week I am going to drive it with the OBD in it and wait for it to start acting up then hopefully I'll see something but if I dont I want to have it acting up when I pull into the shop and have them hook up right away. Hope it works, there could be a wire somewhere getting hot then it acts up. I had a semi truck years ago kept shutting down completely, many shops and tons of money later it was one stupid wire getting hot shutting down the truck, cool it off and it would run again. Not exactly the same as mine keeps running but very poorly, shut it off and sometimes you can go right away other times I have to let it cool down. Real strange.

 

[DonTom]

mine keeps running but very poorly, shut it off and sometimes you can go right away other times I have to let it cool down. Real strange.

Not strange at all, exactly what my intermittent CPSes have done. How many miles on it? If over 100K, change it, even if that is not the issue, but I will bet it is.

BTW, one reason you will not get a code is some codes have to repeat several times to give a MIL. This is to prevent false codes. So you could get the check-engine light soon which will set the code and tell you what the issue is.

Be expecting a MIL with a code of P0320 soon.

-Don- Richfield, UT

 

[Mark_K5LXP]

I'm not sure the engine would run at all without a functioning CPS. The 8.1 has a reputation for those going bad but none I've read about put the engine in some kind of limp mode - they'e no-starts. RVGeorge, it's a slam dunk when you pull codes and it points to a problem but not all problems will set a hard or soft code. If let's say your throttle position sensor is wonky, the ECM doesn't necessarily know the difference between a bad TPS or you're just not pushing the pedal. Watching the gauges via OBD will show you what operating conditions the ECM is actually seeing. There is one for TPS. If you're flooring it and the TPS gauge is at 15% you have a pretty strong clue. Watching timing, mass air flow, fuel trims, load %, and other operating parameters in concert you might observe an anomaly which will point to a particular issue like fuel or timing. If you're not engine savvy then maybe all the numbers and gauges are alphabet soup but that's where you start. Ideally your trusted repair shop would do the same thing, rather than throwing parts at it. It very well may be something dumb like a thermal intermittent but the process is the same for those as a wonky ECM or bad sensor. On my 8.1 I had an intermittent vacuum leak which I observed watching long term fuel trim on the Torque gauge. Came up with a repeatable process of operation that made it happen, literally pulled over on the road, and spraying some brake cleaner found the intermittent fitting. Last year I had a check engine light on my 30 year old pickup and it took about 2 months of daily trips to work watching Torque, a vacuum gauge and a set of status indicators I wired to the engine computer to nail it down to two bad sensors. They tested OK out of circuit but reported values out of range in operation. Never fun to go through all that but such are the joys of running older cars.

Mark B.
Albuquerque, NM

 

[DonTom]

I'm not sure the engine would run at all without a functioning CPS.

They usually become intermittent when they get hot. Did you read about my Y2k RV and my Jeep? They both restarted fine when the engine cooled off.

My Y2K motorhome has a similar engine to the 8.1L but mine is a GM 7.4L

IIRC, the 8.1L is the same engine bored out for larger pistons, etc.

At 100K miles, even a working CPS should be replaced, IMO. While I never did such, it was a mistake not to three times in a row with three of my vehicles. All three crapped out just over 100K or so. A few more miles than that with the Jeep, almost made it to 200K miles, IIRC, before the CPS crapped out.

-Don- Richfield, UT

 

[Mark_K5LXP]

They usually become intermittent when they get hot....They both restarted fine when the engine cooled off.

That's different than what the OP is reporting. He's getting low power.

Mark B.
Albuquerque, NM

 

[DonTom]

That's different than what the OP is reporting. He's getting low power.

Mark B.
Albuquerque, NM

If running smoothly with low power, then I agree. If running rough with low power then I would expect the CPS. Especially if it has more than 100K miles on the engine. And at 100K miles, IMO, it should be replaced anyway. I could have saved myself a lot of trouble if I followed my own advice, but I never did.

-Don- (from the Little Wonder Restaurant in Richfield, UT. Finally a very nice day, where I can check out this town).

 

[Larry N.]

If I haven't mentioned it yet it only has about 55,000 miles on it

^^^From Post #31^^^

Especially if it has more than 100K miles on the engine.

Don, note the above...

 

[DonTom]

^^^From Post #31^^^

Don, note the above...

OIC, 55K.

I would still change the CPS just because of the symptoms as it is not difficult to do and is cheap. Cheap to let the pros do it as well. Less than an hour labor and a fifty buck part.

But if it is low power and very smooth, then I have no idea. An intermittent CPS is unlikely to have smooth but low power.

Intermittents, by far, are the most difficult things to repair in electronics. And not only that, you can never be 100% sure when it is really repaired.

-Don- Richfield, UT