Significant reduction in power

Hello, I'm new to the forum and relatively new to RV's.  This spring I purchased a 1990 Tioga on a Ford 460 chassis.  It's in fairly good knick, but like anything of it's age it has it's issues, which I've been working through with good results.  On a recent trip however, on the way back about an hour from home, I went over a set of cattle gates that were somewhat bumpier than what they looked like from a distance and immediately after I noticed a significant reduction in power.  There was not misfiring or out of the ordinary noises, but it no longer had enough power to maintain a reasonable speed on the highway.  I stopped to check if there was anything obviously wrong that could be seen with the naked eye, however nothing jumped out at me.  Since the engine seemed to run fine besides the obvious lack of power I decided to limp back into town, using the shoulder where safe and my hazards to make it home. 

I've done a bit of research since, and the two most popular theories seem to be a fuel supply issue, either the fuel pump or a clogged filter, or a clogged catalytic convertor.  I'm looking to get people's take on the situation, and their pragmatic advice on how to proceed.  My best guess for a first course of action is to either buy or borrow a fuel pressure tester to rule out fueling.

Could it be as simple s the throttle linkage loose? Check to see if you can open full throttle (with engine not running).

The accelerator still operated as I think it should, I think if the linkage had come loose then there would be not acceleration, not just diminished power.

could be bad O2 sensor or bad computer.

jermc said:

The accelerator still operated as I think it should, I think if the linkage had come loose then there would be not acceleration, not just diminished power.

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I wasn't suggesting it came completely loose. For example, if the throttle is via a cable, one end may have come unanchored. The fact it feels right doesn't mean it is right. However, it was just a suggestion you can consider or not.

Put a vacuum gauge on the engine, that will help diagnose a restricted exhaust.  A muffler, converter or a internal pipe collapse is a possibility.

lynnmor said:

Put a vacuum gauge on the engine, that will help diagnose a restricted exhaust.  A muffler, converter or a internal pipe collapse is a possibility.

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This is a good idea, however, I'm not sure what the base line reading would be if there weren't a blockage, and as the engine still runs it would be a partial blockage, any ideas?

Molaker said:

I wasn't suggesting it came completely loose. For example, if the throttle is via a cable, one end may have come unanchored. The fact it feels right doesn't mean it is right. However, it was just a suggestion you can consider or not.

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I'll have a look at it, but I suspect it would be an all or nothing type scenario. 

butchiiii said:

could be bad O2 sensor or bad computer.

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I'm not sure as I'm unfamiliar with the early 90's 460 engine, so I have a fairly large knowledge gap, but if it were a bad o2 sensor would it not have the engine light on?  And if it were the computer, would it even run at all?

Bad computer would probably go into limp mode. bad o2 sensor "should" turn on the warning
light.

butchiiii said:

Bad computer would probably go into limp mode. bad o2 sensor "should" turn on the warning
light.

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Do you know of any simple tests I can perform that would help diagnose a bad computer?

Bad O2 or other sensor should pop a code on the OBD.... I remember one car I had. When I suddenly lost power going down the freeway I limped off (OBD 1) and did the paper clip bit. read the code on the blinking light and called my Mechanic who said "Emission Control issue"  I popped the hood. Re-connected the MAF sensor and off I drove full power restored.  (Not the first time it happend)

Air filters can also clog and cost you power (Different car)

Loose connections as welll.. Best I can do.

The shaking from the cattle guards may have shook the catalyst loose in the converter. Thus a restricted exhaust. Most muffler shops can check the exhaust pressure ahead of the converter.
No limp mode on the old EEC-IV.

Richard

jermc said:

I'm not sure as I'm unfamiliar with the early 90's 460 engine, so I have a fairly large knowledge gap, but if it were a bad o2 sensor would it not have the engine light on?  And if it were the computer, would it even run at all?

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I've had bad O2 sensor (on other engine type) and it did not throw a code, I found it by accident while changing oil. It had a broken wire.

Dirty fuel filter would be my #1 suspect.  A fuel filter can work ok even though half full of gunk, but a bump or sharp turn can move the gunk enough to block the flow enough to cause problems. 

That vintage of 460 is also notorious for failing fuel pumps. If yours has never been replaced, that would be my #2 suspect. Unfortunately, it's in the fuel tank.  An auxiliary pump inline might help, if the OEM pump is at all functional.

Gary RV_Wizard said:

Dirty fuel filter would be my #1 suspect.  A fuel filter can work ok even though half full of gunk, but a bump or sharp turn can move the gunk enough to block the flow enough to cause problems. 

That vintage of 460 is also notorious for failing fuel pumps. If yours has never been replaced, that would be my #2 suspect. Unfortunately, it's in the fuel tank.  An auxiliary pump inline might help, if the OEM pump is at all functional.

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rls7201 said:

The shaking from the cattle guards may have shook the catalyst loose in the converter. Thus a restricted exhaust. Most muffler shops can check the exhaust pressure ahead of the converter.
No limp mode on the old EEC-IV.

Richard

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These two options are my prime suspects, I've ordered a fuel pressure test kit as it's the cheapest method of diagnosis that I can think of, if the fuel pressure is low, I'll change out the fuel filter first as I don't know when it would have last been done, if the fuel pressure is still lower than ideal operating then I'll look at getting a new pump for the in tank.  If however I find the fuel pressure the first time to be acceptable, then I guess I'll start looking for catalytic converters. Does anyone know if there is any advantage to be gained from upgrading to a higher flow cat?

Why would you not just change the fuel filter without checking?  It's probably 28 years old! Granted, finding and accessing the filter may not be easy, but it ought to be done.

Gary RV_Wizard said:

Why would you not just change the fuel filter without checking?  It's probably 28 years old! Granted, finding and accessing the filter may not be easy, but it ought to be done.

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I may still, and I may change out the cat and the fuel pump for peace of mind as well, but I like to know definitively what a problem is before I start just throwing parts at it.  The previous owners took good care of the RV, and there's been upgrades and evidence of proper repair, so I doubt that the fuel filter is original, but yes, it is something I will change regardless at some point.

Have you put a vacuum gauge on it and checked the vacuum. Should be 16 to 20 and steady also check the EGR valve could be stuck. Check vacuum line to EGR and remove and clean no parts just time. Need more troubleshooting just do not change parts .

sc4668 said:

Have you put a vacuum gauge on it and checked the vacuum. Should be 16 to 20 and steady also check the EGR valve could be stuck. Check vacuum line to EGR and remove and clean no parts just time. Need more troubleshooting just do not change parts .

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Thank you, I was wondering what the baseline vacuum reading should be, I'll test that at the same time as the fuel pressure then.