Help! Is it the Head Gasket?

[Zoro]

I am looking into buying a 1990 RRC. The problem is as follows:

If the RPMs go over ~2000 it starts to run very badly. Under 2K and it is fine, you can drive it all day. But once it passes 2K its over. What do you guys think it could be?

Thanks for the help...

[OkieRover]

I wouldn't say head gasket. But have you checked for hydrocarbons in your antifreeze?
I'm going with something related to the timing or spark plugs, wires, rotor, cap. That would be where I would start.
Is your air idler valve clean?
Do you have a clean fuel filter?

[bartell]

Originally Posted by OkieRover

I'm going with something related to the timing or spark plugs, wires, rotor, cap. That would be where I would start.

Hi Okie,

I would guess at the rotor and cap as well. they are easy and require little effort and they could exhibit these systems.

Other than that, he can try disconnecting the cold start injector after the engine is warm.

After that, it sad to note that his are also symptoms of a dying flapper ECU. These are analogue comuters and over time, the soldering cracks..creating a a buffet of weird running anomalies.

These easiest way to confirm a 4CU is to swap in another. It does not even have to be a known good one..simply one with known (different) symtoms. It only takes a second to swap the plugs. If the vehicle adopts the characteristics of the second computer, you have diagnosed the problem.

Contrary to popular opinion, 4CUs can be repaired. There are 2-3 guys that can do it in the UK. Cost is around £150. One of them can even "reprogram" these ECUs for enhanced performance and dial in adjustment. Used ones, with no guarantee, can be found on ebay.uk.

J.

[atvsmurf]

could be bad converters... hard to check on a test drive, but if you could drop the exhaust, you would know for sure...

[skucera]

My hunch is that the cats are clogged with carbon. The usual way they get that way from bad oxygen sensors. As O2 sensors age, they become gradually less sensitive, so the fuel injection computer compensates by richening the mixture. Eventually a point in the air/fuel map is reached where the engineers knew that the engine would never need to run that rich to run correctly, so the Check Engine light comes on. A previous owner may have disregarded that light, so excess fuel burned in the catalyst matrix, which clogs it with soot.

There are two ways to clear a clogged cat. One is to correct the fault that caused the rich mixture, then let the hot, clear exhaust gasses slowly burn the carbon out, like burning charcoal. This can overheat the catalyst matrices and cause a meltdown, or it might just burn the carbon out. A catalytic converter fire is nothing to laugh at, so I wouldn't recommend clearing the blockage this way. The second way is to replace the cats with new ones. There are nice, fairly inexpensive (under $200 each) cats that flow very freely and can handle Mustang 5.0 HO engines just fine. An exhaust shop (or two) can give you estimates on replacing your cats. Don't worry about getting the exact OEM fitment, because those are hugely expensive, and don't flow especially well by modern standards.

And, yes, there can be other problems that prevent revving above 2000 RPM, but clogged cats are the most common one you're likely to experience.

Scott