92 K1500 stumble/hesitation while driving

[FastGilla]

Hello,

I have a 92 K1500 350 TBI 5 speed Ex. cab. I have been dealing with drivability issues for a while now and I have read dozens of forum posts with similar symptoms with no help as to fix this problem, so as a last resort, I am making a post myself and ask for some help.

The problem is with drivability. The truck will start and idle fine and will idle all day long if you let. It also is showing no check engine lights on the dash. The problem is when you start driving, around 2000ish RPM, the truck will have low power and sort of backfires as you try to rev through it. I've also noticed it doesn't like hills any more as I have to down shift usually to second to even get to the top.

This all started after I pulled the engine to replace the clutch, rear main seal, oil pan and fix a couple other small issues. There were no driveability issues before I pulled the engine and I never removed the TBI unit, distributor, or intake. While I had the engine out, I also put in a set of mid length headers with a new O2 sensor and I built all new exhaust for it. Not sure if this is relevant or not, but it no longer has a Y pipe. I didn't put in an X or H pipe either, it is just 2.5 inch mandrel bends and straights to mufflers and it dumps just passed the axle. I have replaced a couple sensors that broke and the fuel filter. What I am wondering is if the problem is as simple as the fuel pump is going or did the headers mess with the PCM and its a tune issue? Any guidance will be greatly appreciated.

Thanks,
-Brandon

 

[Erik the Awful]

Check to make sure all your ground wires are connected, clean, and tight. Check the wire to your new O2 sensor and make sure it's making a good connection.

 

[DerekTheGreat]

Why do people say stuff like "..no check engine lights?" There is only a ONE check engine light. Sorry man, it's not just you but all the ads I come across on Craigslist that have that mistake in them, yikes..

Anyway, I'd want to look at the data stream but my finger is already pointing towards the headers you put on. Factory one wire o2 sensors don't like them, they don't get or stay hot enough. And since you've said it starts and idles fine but has problems when running, definitely seems to hint in that direction.

 

[red98]

Why do people say stuff like "..no check engine lights?" There is only a ONE check engine light. Sorry man, it's not just you but all the ads I come across on Craigslist that have that mistake in them, yikes..

Anyway, I'd want to look at the data stream but my finger is already pointing towards the headers you put on. Factory one wire o2 sensors don't like them, they don't get or stay hot enough. And since you've said it starts and idles fine but has problems when running, definitely seems to hint in that direction.

Usually means that no WARNING lights are on, there's definitely more than one warning light on most trucks, mine has a CHECK GAGES light as well as the engine light, I imagine there are some I'm not thinking of as well.

 

[Schurkey]

Schurkey's Perpetual Advice:

What is the fuel pressure under load, or with the relief button on the fuel pressure tester pushed so fuel is flowing out the clear tubing?

Connect a scan tool, look at the data stream. I'd especially want to see short- and long-term fuel trim, knock sensor activity, coolant temperature, MAP sensor reading.

How long has it been since the usual "tune-up" services were performed? Test cranking compression pressure, distributor cap/rotor condition, initial ignition timing, plug wires, spark plugs, air filter, fuel filter, etc.







In addition,
Is the AIR pump (if equipped) diverting to the air cleaner when it should? If it's not diverting, it'll screw up the O2 sensor so that it provides a false-lean indication.

TBI distributors have a known problem with the magnet on the mainshaft. The magnet cracks, provides weak/erratic signal to the ignition module.

 

[FastGilla]

I put a new cap, rotor, coil, plugs, wires and air filter on when I put the engine back in the truck. I have also made sure the firing order is correct at least 3 times since I have doubted myself on multiple occasions. Fuel filter was just replaced about a month ago as an attempt at finding a solution. I've never checked compression on the engine, but I guess it doesn't hurt to check. The engine had no running issues at all before the clutch and headers install so I wasn't thinking there was any compression loss. Same thing with timing, I've never put a light on it to check or even loosened the distributor in the truck since I've owned it.

I will try to check fuel pressure this weekend and report back. I don't have a scan tool or know anyone who would have one for anything other than OBD2 vehicles, so I will have to look in to that.

Would it be wise to install a heated O2 sensor and wire it up accordingly?

Thanks for the advice so far, I really appreciate it.
-Brandon

 

[Schurkey]

Would it be wise to install a heated O2 sensor and wire it up accordingly?

The problem with the truck is not the lack of a 3-wire O2 sensor. A 1-wire sensor IN GOOD CONDITION will work just fine.

Having said that, the next time I'm due for an O2 sensor on my '88, I may consider a 3-wire. My truck drops out of closed-loop operation at idle because the sensor cools off. But I'm back in closed-loop before I can get across the intersection when the light turns green.

 

[LC2NLS6]

Check fuel pressure first. And do it under load, rev it up, if it drops to 7-8-9 psi, new pump and/or inspect the wiring and grounds to the pump for proper voltage. Easy to hot wire it off a new relay if need to. Then verify timing.

 

[LC2NLS6]

How did you remove and install the engine with the distributor cap in place, yikes, maybe the cap is cracked?

 

[FastGilla]

The truck has a 3 inch body lift and I also had the entire core support out because the body mounts rotted out up front. I didn't have to lift the engine very high and pulled it straight out the front. I will double check the cap and replace if needed.. Thanks for the tip.

O2 sensor is was new at the time of headers install... There was no way the old sensor was coming out of the manifold, nearly 30 years of Wisconsin winters made sure of that.

-Brandon