Advice for next steps -- 4.0 issue

[mazdab40004x4]

I've got a 94 B4000 that seems to have a few issues and I'm looking for some input on the next steps to take.

Symptoms:
-idling poorly
-seems to chug when cold and stumbles/misses when warm
-struggles to get up hills, especially when cold, and revs drop significantly on incline
-backfire from tailpipe occasionally when struggling up a hill and giving gas
-poor gas mileage
-exhaust smells 'rich' -- like unburned gasoline
-No CEL
-No coolant loss or sweet smell

It looks like a previous owner sucked some muddy water into the intake at some point -- there was mud caked on the air filter and the intake hose was lined with georgia red clay.

So far I've cleaned and rejuvinated the air filter (it's a K&N drop-in), cleaned the MAF, and cleaned the throttle body with no improvement. When cleaning the throttle body, I looked inside the upper intake and saw a whole bunch of oily sludge and liquid in there.

I know it just plain needs a few things including plugs and wires and a new thermostat (it's stuck open). But I'm wondering where to go next to fix this problem. Do I remove and clean the upper intake? Replace the PCV? TPS? IAC? Anything else, and in what order?

 

[Big Jim M]

The very first thing that NEEDS to be done is a compression test.. That will tell you and us if you have a mechanical problem or simply if a good tune will fix her up.
Big JIm

 

[mazdab40004x4]

OK, thanks. I've not done a compression test myself before and I do not have the tool so I'll have to see if Autozone rents them. I've read the Haynes info for performing the check and it seems doable for me.

Haynes does not list a specific compression reading -- they just say lowest reading must be within 15 psi of highest reading and all must be higher than 100. What would a 'good' reading be?

 

[Big Jim M]

Good?

OK, thanks. I've not done a compression test myself before and I do not have the tool so I'll have to see if Autozone rents them. I've read the Haynes info for performing the check and it seems doable for me.

Haynes does not list a specific compression reading -- they just say lowest reading must be within 15 psi of highest reading and all must be higher than 100. What would a 'good' reading be?

I'd take 140/150 on a warm day with a nearly hot engine, properly done.
What to look for would be one or more sylinders with 90/100 or even less compared to the high readings.
If the thing does read good then look to a complete tune of the engine.
That Georga red clay bothers me a little. BUT the problem could simply be the previous owner used a K&N or similiar filter instead of the tried and true factory type. When driven on red clay dusty roads the small red particles BYPASS THRU the K&N and get into the engine..
Big JIm

 

[mazdab40004x4]

Yeah that K&N filter was CAKED with clay when I got it. I banged about 2 pounds of it out of the filter. To be honest I'm surprised the thing even ran at all with it like that, but it did. Cleaning it made it run better, but not great. When I do a tune I'm planning to put back a stock filter.

Thanks for the compression info. Looks like I have yet another weekend project.