When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Recently it has begun stumbling under load. Seems to run fine at idle, and if you are sitting in park you can rev the engine without issue.
Things already replaced:
Fuel Pressure Regulator (30 PSI at idle - 38 PSI under load) I have installed a digital fuel pressure gauge off the fuel rail so I can monitor while driving. No change in pressure when stumbling.
Fuel filter
EGR Valve
EGR Valve Position Sensor
EGR Valve Solenoid
Coil
Spark Plugs
Spark Plug wires
Distributor
Ignition Control Module
Engine Temperature Sensor
Catalytic Converter
In-tank fuel pump (about 4 years ago).
Idle Air Valve
The computer is throwing no codes - get an 11 for both Key On Engine Off and Key On Engine On tests using ODB1 reader.
30 seems low for idle. I think mine is around 40-50. If your not pulling a lean code then I'm not sure what's going on unless it's a timing issue. You may have a timing chain with quite a bit of slack. You may also need to check your injectors.
Yeah the modern ethanol gas is horrible when it sets for awhile like on motorhomes. Do you have your original regulator? You may wanna stop by your local NAPA and see what they pull up for the replacement. They are 'sometimes' knowledgeable on which part you need.
With all the conversation about a FPR etc possibly being the wrong part I'd shy away from RockAuto, NAPA or any other outfit opting instead to get the original part through a dealer based upon the VIN. Some automatically think dealership's charge double or triple what the aftermarket suppliers change but a bit of shopping for the correct part number often yields great results, major cost savings IOW.
Of course all this depends whether the correct part number is known.
Well, I replaced the fuel pressure regulator twice tonight. One to put in CM-4646. It generated pressures almost identical to CM-4764. Then I re-installed the OEM regulator that came with the vehicle. It generates pressures a good 10 PSI higher than the two new replacements.
And I still have my stumbling problem.
So, my stumbling problem is not a result of replacing the fuel pressure regulator.
So tonight I replaced the MAP sensor. It is very easy to get at - it's mounted on the lip of the rear flange of the engine opening, above the blower motor.
Took it on a test drive. I go in a loop - about 15 minutes of 60 MPH, followed by about 20 minutes of interstate, followed by about 15 minutes of surface streets.
At first I had it licked. While the engine was cold, it did not stumble any that I could detect. But after running on the interstate and then merging onto the next stretch of interstate, it was bucking and popping and backfiring.
At interstate speed it runs fine.
I was down to less than a quarter tank of gas. I put in 2 bottles of GumOut Fuel Injector Cleaner. Went for another circuit. Had some more stumbles. Stopped at Costco and put in a full tank of their Top Tier detergent gasoline (I usually use Mapco which is not Top Tier gas).
Went home - had a few more minor stumbles but nothing major - all surface streets.
Got home, sat in the driveway and revved the engine a few times - blew a heater hose. Nice ending to the evening.
Starting to think my dad is right - best fix for this RV is a stick of dynamite!
Still original ECM. I picked up the second replacement today. I'm gunshy of opening the box given how crappy the first one was - hoping I can return this one for a full refund since the seal is not yet broken.
Well, the problem is once you open the box you can only return it for core credit ($70).
I'd hate to install the new computer and find it runs like crap again like the first computer did and only be able to return it for core.
But this weekend I'm replacing the last of the engine sensors - the Throttle Position Sensor and the Air Charge Temperature Sensor.
About the only other sensor on the vehicle that may be engine-related that I have not replaced is the Speed Sensor in the transmission.
I'm also going to build a "smoke can" this weekend that vaporizes mineral oil (baby oil) and pump it into my intake manifold to see if there is a hidden vacuum leak somewhere.
If I have time and that doesn't get it I'm going to pull the injectors and flush them out and replace their o-rings.
So, this morning I replaced the Air Charge Temperature Sensor, and voila, the problem is fixed!
In hindsight, I probably should have suspected this sensor earlier. About 6 months ago the engine was having difficulty idling when cold. When I had it in the shop to have the Distributor replaced, I told them this, and they replaced the Idle Air Control Valve. It did not help. Since it only idled poorly when cold, I suspected a temperature-related issue, so I replaced the Engine Temperature Sensor. It immediately fixed the cold idle problem.
It turns out, the Engine Temperature Sensor and the Air Charge Temperature Sensor are essentially the same sensor and sensor body, but on the ACTS they have machined away the brass nipple end of the sensor body to expose the actual sensing element to the air to improve response time.
I suspect that I cooked both these sensors with the extended overheat condition I had back in 2016. If one sensor got cooked, it should not be surprising that the other sensor got cooked also.
Of course it is obvious how primitive the ODB1 EEC-IV computer system is that it is unable to detect faults in these critical engine management sensors. In both cases, no code was thrown due to a faulty sensor.
For anyone in the future who might stumble across this thread, I'll admit that my approach to fixing the problem - sequential random replacing of various engine sensors - is not the smartest or most cost-effective way to do diagnosis. The proper way is to identify the sensors that might be relevant and test them. Most sensors can be tested by some procedure that can be discovered in a shop manual or online.
I did not mind the "throw parts at it" approach so much for a few reasons. First of all, at the time of this writing the vehicle is 29 years old. Since I have already had sensor component failures, I don't mind replacing sensors that are 30 years old. I do worry a bit about replacing possibly good OEM components with aftermarket components that might not last as long, but after all, this is an RV, so it's not going to see that many miles in the years ahead anyway. It only has 86K right now, so it's averaged 3,000 miles a year over the last 29 years.
The second reason I don't mind my approach is that most of the sensors are relatively cheap. I suspect for all the sensors I've replace so far I'm in for less than $300. And the labor to get at the things to test them is about the same as to get at them to replace them, so if I'm going to go to the trouble, I may as well just replace the thing rather than figure out how to test it.
The final reason goes hand in hand with the second reason, and that is that I live in a subidvision with an HOA, and I can't leave my RV in the driveway for more than 24 hours. So time is of the essence when I start pulling things apart. It's quicker if I'm going to pull the sensor for testing anyway to just pull and replace.
So, until the next thing breaks, see you 'round the 'net!
