Running terribly

someotherguy said:

Welcome -

Before checking anything else you should look at all your engine bay grounds. Bad grounds can cause lots of issues. Battery to engine, also battery smaller wire to the passenger fender, 2 straps at passenger rear of engine - one from cylinder head to firewall by the heater hoses and other from same spot on firewall down to frame, 2 at the thermostat housing stud, would be main ones to concentrate on.

It may be a vacuum leak somewhere else, causing the code 33. You've eliminated the vacuum line from the TBI to the MAP sensor which would've been my first suspect. Other somewhat common places would include the throttle body base gasket, and the throttle shaft itself can be loose enough to be a leak. Intake gaskets are another possible source. You can easily check for vacuum leaks with an *unlit* propane torch using the small skinny bottle and an angled nozzle. Get it running and turn the nozzle on a little bit and poke around until you notice a change in how it's running, and you've found the area of the leak.

Also highly suspect is the CTS (coolant temperature sensor) at the front of the intake near the thermostat housing. These go bad with age/use and are often the cause of running very rich; it won't cause your code 33 and generally they go bad without setting the related codes, at least in my experience. You can test it with a multimeter pretty easily by simply unplugging it and checking resistance across its two pins and compare the reading to the engine's actual coolant temp, measured with an IR thermometer, scanning around the thermostat area until you see your highest reading. CTS is relatively inexpensive and really should be considered a periodic maintenance item on these trucks. The higher the resistance it reads, the colder the ECM thinks your engine is, and will richen up the mixture to match. They usually fail reading far higher resistance than they should.

Also, confirm base timing set to zero with engine warmed up and timing bypass wire (tan/black) disconnected, as someone may have it set wrong. On your '90 the tan/black wire disconnect should be behind the plastic electrical convenience center cover on the passenger side firewall; look near the bundle of wires going down towards the engine and you'll see it.

Richard

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Jeepwalker said:

And yeah, ohm-out the temp sensor (cold and hot)...as was suggested above, and yeah, make sure the grounds are good (current goes from Neg -> Pos).

If the temp sensor is good, I think I would probably start from the true 'basics' troubleshooting since you don't know the history. Eliminate things. Perform a compression test (vehicle warm, 5 puffs each cyl), vacuum test (although it might not be accurate at the moment), fuel press test. While you're warming the engine up prior to doing the compression test, sniff the radiator for combustion smells (head gasket), and bubbles. While you have all the plugs out for the compression test, obviously inspect each plug and note the cylinder each came out of. Put the plugs on the wires (not installed but touching the engine), have a helper turn the engine over and you verify they're each firing, and inspect all the parts carefully (plugs/wires/cap/module/dist pick-up can go bad). Make sure the dist cap isn't cracked and wires aren't jumping spark to each other. Write down each cylinder's compression. If you don't have those test tools, you can usually rent for free from like Autozone or OReilly's. Y/T videos show how to perform the tests. Or buy for cheap from H/F.

Run some fuel into a glass jar and let it sit ..verify for water. A good slug of water from a gas station can make a vehicle run like true crap and it can spoof all sorts of serious engine problems, when the only issue is water in the fuel. It can take a couple tankfulls and several can's of HEET to get a bad case of water out of the system. The filter should be changed if you haven't already. I once (years ago) bought a pretty nice car super-cheap because it ran like crap and the dealer couldn't figure it out, thought it needed major engine work. I did too, but hey... I bought it cheap enough, right? After filling up at a gas station and a can of HEET (on the way home) 3 miles down the road it suddenly smoothed out and ran perfect!! There's no code for 'water in the gas' and it has caused a lot of guys to replace parts and scratch their head for days/weeks. So you want to cross this and other 'basics' off your list.

My '89, the star (reluctor) deep inside the distributor corroded to pieces and ran like crap. Literally a couple blades rotted off! The pickup was all corroded too. Hard to believe it ran at all! Even if those things are good, there are two very fine wires that go to the pick-up coil which can fray, or short/corrode internally. You can't see these parts and there's no code that'll point it out to ya. It's just a matter of crossing the basics off your list. Ya have to pull the distributor, remove the dist gear to pull out the dist shaft (out the top of the dist) in order to inspect those parts. Not difficult to do. Took me a long time to find that issue on my truck, ran perfect after that fix. Like I say, yer not going to get a code for something like that. That's why I say check for erratic spark. If you replace them, don't buy cheap import replacements. Buy Standard Parts, or AC Delco ..a 'good' brand. It's not a place to go cheap.

Good luck. Seems like you snagged yourself a nice truck! Let us know what you're finding. We'll help you get it running like a champ again!!

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Jeepwalker said:

Note... if you have a scanner, you probably don't need to ohm out the temp sensor. Just observe the reported temp before starting the truck, does it match the outdoor temp? Once it's warmed up, estimate engine temp with an IR temp gun at the coolant outlet on the intake, should be close to the OBD1 reading. You can still ohm the sensor out if you want to ..only takes a short time ...then you compare the ohms you got (cold & hot) to a temp/ohm chart for your year truck. There's videos how to do it if you haven't done it.

There's probably videos how to verify the distributor reluctor and pick-up too.

When you remove the fuel filter, take care not to tilt it. Cap the ends so the gas doesn't spill or drain out. Once removed from the vehicle, then drain the fuel into a glass jar. Sometimes water will collect in the bottom of the filter and you'll see it when it settles out in a jar. If there's water in the fuel.

I didn't get into vacuum leaks. You can put a 14" length of heater hose on the end of a propane torch (unlit) and go around the upper regions of the motor. If there's a vacuum leak, it'll suck in the propane and run smooth for a second or two. Short bursts of starting fluid or carb cleaner, here and there will do the same thing. Inspect all the hoses real carefully.

What happens when you unhook the hose from your EGR? Does it run better? Did you clean all the carbon out from under it with a screwdriver (and the intake)? Does the bellow (underneath the EGR) move up/down freely? It's supposed to bleed off vacuum. Have someone start the vehicle and meanwhile put your fingers under the EGR and feel for the diaphragm to lift. If it doesn't 'lift' there's a problem, Maryland ...lol

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Schurkey said:

1. Vacuum tubes, and vacuum hoses are very different things.

2. If the MAP is showing low vacuum, the computer will respond by adding fuel. It thinks the engine is under load.

3. Misfire will cause the O2 sensor to send a false "lean" signal to the computer. The computer responds by adding more fuel.

First Guess: This thing needs a COMPLETE, THOROUGH "tune-up"; including having a scan tool connected to view the data stream, cranking compression test, perhaps a cylinder balance test. I would verify fuel pressure.

How old is the O2 sensor?

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Sle-z_90 said:

I was using hose and tube interchangeably. Didn’t realize it referred to different items.

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