Hey guys, new to the forum here! Having an issue and could use a little help. Truck starts and idles fine. Runs fine for a bit but once it is warmed up if I accelerate hard it lurches and stutters a bit. Then it slowly gets a little worse until eventually it won’t accelerate and will stall. Wait a few mins, fires up and runs fine for a bit until it does it again. Have recently spent a good amount of time replacing:
MAP sensor, PCV valve, spark plugs, distributor, EGR valve, TFI module, EGR sensor, high pressure fuel pump and filter. Runs a bit smoother but didn’t fix the issue. Got a code 33 on OBD1. Checked voltage on TPS. Got 5v on first wire, second wire just under 1v, then responsive with opening throttle all the way up to 4.5v when fully open. Leads me to believe it’s not that. Any other suggestions? Or anyone else had this issue?

 

Welcome to the forum

So it runs fine when cold, Open Loop, when system is running air/fuel mix from tables in memory
Then issue comes up after computer is doing on the fly air/fuel mix with O2 sensor feed back, Closed Loop

From description, yes the TPS would be first guess as that sensor is used to give the computer an instant "heads up" to add more fuel so engine doesn't stumble, its what the Accelerator pump did on a carb engine
The MAP sensor is just too slow to respond for instant throttle response

TPS looks like it checked out OK, no jumps or drops in voltage when moving it thru it full motion?
Needs to be a steady increase as throttle opens, jumps or drops will cause stumbling(dead spots)

And SPOUT connector was reconnected after setting spark timing at 10-12deg BTDC?
SPOUT signal is from computer, it tells the TFI module when to advance and retard timing based on throttle position, it does what vacuum advance did on older models
TFI module can only do RPM advance

Spark advance actually needs to be less when accelerating as the richer fuel mix burns faster, if just RPM advance is used stumbling will be the result when accelerating


33 EGR did not open/ respond during test or if memory code, did not open intermittently
EGR issue, stuck open would cause idle issues, not working would cause ping/knocking when accelerating

There is an EGR sensor
In 1986 it may be on the EGR valve, top
When computer uses the EGR solenoid to send vacuum to the EGR valve, the sensor will give the computer feedback on the internal motion of the valve, not a very accurate system, lol
It was changed to a pressure feedback system in later years

Code 33 means the sensor was not responding when computer opened EGR solenoid and sent vacuum to EGR valve
This could be from intermittent solenoid or sensor
Solenoid gets 12volts with key on, the computer uses its ground connection to solenoid to pulse it for a little or alot of vacuum passing thru the valve

EGR is used on acceleration AFTER engine warms up, closed loop
Too much exhaust gases could cause stumbling and too little pinging/knocking
Unplug the vacuum hose on EGR valve and plug it
After engine is warmed up see if no EGR use drives better(may have some pinging), if you still have the stumbling then EGR is not the issue

Also a can of Seafoam, or similar injector cleaner in the gas tank to clean injectors tips can help a bit, but not your issue, just good to clean the tips, better MPG

 

I do not just go throwing parts at a problem but when I buy a new to me truck, I dont think I have ever bought a truck that has not had the TPS sensor changed. That is one part I will replace along with a new O2 sensor along with a complete tune up kit. One time I had a faulty TPS and I disassembled it and was able to clean the contacts and get it to pass a resistance check. I had replaced the old TPS with a new TPS and did fix the truck. Just for giggles I re installed the TPS I had cleaned and tested and the problem re appeared. Replacing O2 sensors and TPS is just part of my first tune up for a new to me truck.

In one mile how many times are you on and off the throttle? 10 times easily if not much more then that, and a truck with 100k miles that is over a million times you have cycled the TPS.

Seafoam is your friend and it can smooth things out. Here is how I clean and a cheap test to do on injectors https://www.ranger-forums.com/member...2/#post2201856

 

Update: pulled EGR off and deep cleaned passage going into EGR, in case it was clogged and stopping EGR from opening. Replaced TPS since it’s basically the only thing I haven’t replaced at this point. Still getting the same issue. Really beating my head against a wall at this point. One other thought is clogged/ broken filter in the cat causing buildup and lurching. Any thoughts on this or other things I might not be thinking of?

 

Vacuum gauge can ID partially blocked exhaust, but wouldn't expect surging, just lower power at higher speeds

Test EGR but putting a vacuum hose on it and sucking on the end of the hose, as EGR opens idling engine should start to stumble

Check spark timing, should be about 20-22deg warm idle, with SPOUT connected, 10-12deg without SPOUT
REV engine, should see timing change, retard first then advance with SPOUT connected

 

As far as the Catalytic Converter, you dont have a down stream O2 sensor, so the computer wont throw any codes with converter off of it as it only knows what goes past the sensor. That would let you know if converter is clogged or not. If not clogged just re install converter.

Newer trucks with a down stream sensor will not see a difference from the up stream senor as the converter changes the fumes. Computer compares the reading of the two different sensors. Older trucks dont have a down stream sensor.

I am not familiar enough with the EGR on these trucks to be any help.

 

Any update on the issue? Having identical issues with my 86’, about to replace the EGR valve when a new one comes in tomorrow. Truck idles fine then loses all power if you accelerate too hard after a minute or so of driving, but I’m a bit worried even if the EGR valve seems to look carbon’d up that it wouldn’t cause problems this extreme so hoping to hear what you did to fix your issue.