Diagnose my 91 EB

[sd_iconoclast]

I have a '91 EB 5 spd with about 175K on it. It has developed some strange symptoms lately. I am hoping one of you guys can recognize them.
I bought this car with about 135K on it. It had a cracked head. I replaced the head with a new one and I replaced virtually everything easily accessible with the heads off. This included alternator, all AC components, fuel injectors , radiator, hoses, water pump, Ignition wires and plugs etc. Then the other head cracked so I replaced it with a new one and replaced the lifters at the same time as I seemed to have alot of valve clatter at startup. This clatter responds really well to rislone, but has not gone away completely.
Recently, the car has developed a tendency to "shuddder" under fairly heavy load. I can get it to happen when I am accelerating and I shift up. It will also happen when I am going up a hill and I need to downshift. It seems like reducing the load will make it stop.
The other day, my son was driving it and experienced one of these "shuddering" episodes but he did not let off the gas. He says he heard a load "pop" and then a backfire. Since then, the oil pressure has been low. It used to idle at about 15 psi at around 700 RPM. Now it idles at between 6 and 9 PSI. I see these pressures on my equus mechanical oil pressure gauge. The factory gauge jumps around quite a bit when I am in the 6-9psi range.
I have also noticed that I now get a noticeable "clattering" noise at about 3000 RPM. But, the car runs pretty well if I just keep it below 3000 RPM and 3/4 load.
I also try to rev the engine at idle to keep the oil pressure up.
Any ideas?

 

[1991 sploder]

Sounds like a clogged fuel filter and possibly clogged oil feed.

 

[1991 sploder]

Check fuel pressure 700 is low idle too u wont have good oil pressure.

 

[sd_iconoclast]

Check fuel pressure 700 is low idle too u wont have good oil pressure.

I have been thinking of changing the fuel pump.
I have changed the fuel filter since I got it, but I think it has the original fuel pump.
What RPM should it idle at?

 

[1991 sploder]

Cant find a direct awnser on that just that its based on tempurature by the idle air control valve but i adjust mine off the throttle body but mines at like 1200 when I first start and about 850 when the truck is hot.. Low idle causes clatter for me too. Switch to 5w30 chiltons says preferred over 10w.... Is this a manual?

 

[sd_iconoclast]

The 1200 when cold sounds right, although it seems to hunt around until it warms up.
Mine definitely idles between 600 and 750 when warm though.
Yes it is a manual. it is a 5 speed.

 

[1991 sploder]

Is it possible you have a warped flywheel?

 

[Mbrooks420]

Why would a warped flywheel effect oil pressure?

 

[1991 sploder]

Why would a warped flywheel effect oil pressure?

Haha it wouldn't but the shuttering in gear when you shift gears while driving when under a load is probably a clutch issue. Could have a separate issue.

 

[Mbrooks420]

Once the clutch has fully engaged the flywheel I can’t see how it’d shutter. I could see it being an issue as it slipped, but not after being engaged.

First step is to figure out the oil pressure problem. The shudder will be moot when the motor gets toasted due to lack of oil.

 

[1991 sploder]

Yes it's a issue because its probably slipping causing the shuddering.

car has developed a tendency to "shuddder" under fairly heavy load. I can get it to happen when I am accelerating and I shift up. It will also happen when I am going up a hill and I need to downshift. It

How is that an oil issue? Oil on the clutch and flywheel maybe.

 

[1991 sploder]

 

[sd_iconoclast]

I went ahead and replaced the fuel pump and filter. It seemed to affect very little except that my oil pressure problem seems to have gotten better.
My aftermarket gauge shows 10psi at idle but I never see the stock gauge go to zero and my idle never goes below 750. All the other problems remain though

 

[sd_iconoclast]

I convinced myself that this was an egr problem. Then I found that my 91 eb does not have an egr valve

 

[sd_iconoclast]

I then decided this was a problem with the MAF sensor. I replaced it. No change.

 

[sd_iconoclast]

My son experienced a big " shudder" on the freeway and got a check engine light. I read the code and it was 41, O2 sensor always showing lean. So I just ordered a new O2 sensor from RockAuto. It should be here tomorrow.
I think my current O2 sensor is only about a year old but it might be as much as 3 years old.

 

[1991 sploder]

I would think it's probably something other than the 02 sensor.. Probably a vaccum leak....

 

[410Fortune]

Yes if the 02 sensor indicates lean then chances are the 02 sensor is doing its job
Check for cracks in the intake tube between the airbox and throttle body
Check for any cracked/broken vacuum lines, also the EVAP line from the charcoal canister to the throttle body.
Check for intake leaks, air leaks at the base of the metal upper intake, you will see a buildup of soot in the area if the gasket is leaking

The gen I trucks with their 3 piece intake tube or the early one piece intake tubes are known to crack in the accordion sections

A "pop" or backfire through the intake is almost always a lean condition
Lean can be not enough fuel OR un metered air entering the system
UN metered air is air the mas air flow sensor did not see and the computer is not dumping enough fuel for the amount of air entering the combustion chamber. The lean code means that the computer has seen the extra air by way of the 02 sensor and has attempted to add fuel to compensate and has been un able too long enough to set a code.

Check fuel pressure, a new pump and filter does not mean you have good fuel pressure. Fuel pressure NEEDs to be tested at the fuel rail. You should have 36 psi +/- at idle..... a 1991 explorer fuel pressure vacuum regulator has been known to fail after 29 years

Computer codes are your best friend they tell you exactly where to look.
Although a 1991 02 sensor with 175K on it is likely to be very lazy, I think in this case the code indicates the 02 sensor is doing its job. Nevermind I see your current 02 sensor is 3 years old.... hopefully its a bosch. 1991 has what 2ea 02 sensors?

Usually a failed 02 sensor code will be something like "circuit failure" since they are heated and as everyone knows things that heat up do not last forever

 

[sd_iconoclast]

Thanks for the detailed recommendations. I will hook my YouTube smoke machine to it to look for vacuum leaks and report back.
Only one O2 sensor on a 91.

 

[1991 sploder]

Just use like a vaccum tester or something on each of the vaccum diaphram lines... Disconnect each line on the octopus on the intake manifold..