86 2.5L Running Terrible When 'cold'

[87Warrior]

Truck in question: 1986, 2.5L TBI, AX5, 207. 99% stock, 168k miles.

 

As of lately I have begun having driveability issues with my truck. When ever the air temperature is below 35 degrees it runs terrible for the first 10 miles or so.

 

Example:

Cold start first thing in the morning it fires right up and has a slightly elevated idle. The engine responds very quickly when throttle is applied.

 

Things go down hill when the truck hits the road. Put the truck in 1st and notice some hesitation/stuttering as I head up the driveway. Once on the road I can only manage 45-50 mph in 3rd gear. The truck will simply not go any faster. If I try to go faster I am met with a bad engine stutter. It sounds like it is either not getting fuel or spark. Depress the clutch and the engine will rev to the moon like normal.

 

While holding an inconsistent 45mph with a slight stutter, the truck will occasionally pick up speed as if somebody hit the 'go baby go' button. I am able to reproduce this by varying the throttle position between 1/4 and 3/4 throttle.

 

After fighting with the truck for 10 miles, it is as if somebody flips a switch. The truck will instantly run like normal and will let me achieve any speed the 2.5L can muster.

 

When this issue started to appear, fuel economy went from 20+mpg to 13-mpg. The truck will also diesel for 1-2 seconds when shutting it off. This only occurs at the first start up of the day. Let it sit for a few hours and it runs like normal, however it will occasionally diesel at shut down.

 

My fuel tank, fuel pump and fuel filter are approximately 8 months old. Air filter is clean. I am running 87 octane fuel from the same fuel stations I have been going to for 15 years.

 

I think the truck is running pig rich for whatever reason and flooding it when cold. I am unsure how to go about diagnosing the TBI Renix system. What sensors, if any, does the Renix system read from when in closed loop?

 

My gut is telling me to replace the TPS and TBI injector. However, I don't want to start blindly throwing parts at it.

 

Thanks for any help :)

[HOrnbrod]

Does the temperature gage show normal transition times from cold to normal operating temperature on the gage? If so the first thing I'd change is the O2 sensor. It's taking way too long to start passing traffic to the computer to start the open to closed loop change.

[87Warrior]

This truck has always run a bit on the cool side, about at the 1/2 way point between 100 and 210 (as indicated on the factory gauge). Regardless, the gauge seems to read just as it did in the summer. I can watch the gauge climb and fall some when the t-stat opens. Winter or summer it occurs at about the same bend in the road every time I leave the house.

 

I know this truck sat unused for at least 8 years in an unknown condition. I am not sure how the sensors handle that kind of condition.

 

Is there anyway to test the O2 sensor on the 2.5l TBI?

 

[HOrnbrod]

If you have a 4-wire O2 sensor (I think the 86 does), about the only thing you can do is verify it is receiving the switched 12V to the O2 heater circuit and verify the heater coil inside is not open. For the signal out to the computer there's not much you can do w/o an A/F meter or similar to monitor it. If your state has emissions testing you could ask them to test it for you.

[Guest]

Like Don suggested (if you haven't already) replace the O2 Sensor; at the very least have it checked out. That little bugger is a huge catalyst for the 2.5 to run properly. I know when I got around to replacing mine, It fixed most of the stumbling and bumbling my 87 had when trying to get up to speed, as well as fluctuating idle at a stop or warming up in the morning.

[cruiser54]

Another huge issue for the sensors to work properly is the ground strap from the firewall to the head stud. Make sure it's there, and if it is, remove the bolt from the firewall and scrape the paint from under the strap. Clean the strap and bolt and reattach.

[dasbulliwagen]

I agree with the O2. Mine did the same thing for a long time after I bought it. It also sat for many years. I tried all the tune up stuff, fuel cleaner etc etc.... then popped a new O2 in and it ran great ever since. The factory had an 80000 mile service interval for them, so its probably due anyways.

[mjeff87]

X whatever on the O2 sensor......had the same problem on mine at about the same mileage. Disconnected the sensor after trying everything else under the sun to fix it with no change, and the stumble went away. Like yours mine happened about 5-10 minutes into driving it and then it ran fine. Try unplugging it and running it for a day and see what happens.

[87Warrior]

I was forced to put the top on my TJ in order to make it into town to order an O2 sensor. (It was below freezing, I'm not that tough) New O2 will be at the local NAPA in the morning.

 

I am sure the O2 sensor needed replaced anyhow. Let's hope it fixes problem :)

[54bobby]

I was forced to put the top on my TJ in order to make it into town to order an O2 sensor. (It was below freezing, I'm not that tough) New O2 will be at the local NAPA in the morning.

 

I am sure the O2 sensor needed replaced anyhow. Let's hope it fixes problem :)

did the new o2 sensor fix your problem?

[87Warrior]

The old O2 sensor was a bit, well, missing:

 

The new O2 sensor cured it about 95%. I haven't run a full tank of fuel through it, but it seems my mileage has gone up. General driveability seems to have improved as well.

 

The truck has had a very slight stumble at about 1/2 throttle when stone cold. This goes away after a few minutes. Also, after a hard drive (i.e. into the wind at 70 mph) it will diesel briefly when shut down immediately after.