Trouble Shooting Help

[matt s]

88 suburban. Trying to bring it back to life. 350 tbi. Sitting for 5 years. Ran tip top when parked.

Here's the deal. We got it running and it idles great and in neutral or park revs all the way up nice and smooth. Put it in drive and it can barely even go. It just doesn't make power or spool up. Sputters, coughs, backfires a little bit. Stinks like hell too. When it first started it wouldn't run well and shot a ton of crap out the exhaust. Big chunks of rusty garbage. I managed to get it up to 35 and drive it over to my parents place but that was it.

Bad Gas? Likely, however tank was mostly empty and we filled it and added a bit of heat (yeah I know heat is snake oil). That and the fact that it idles and revs great makes me think the gas is ok.

Fuel pump? I really don't want it to be this. Pain in the rear as it's an inner tank pump.

Fuel Filter? I just don't ever get that lucky. It's never that easy.

Plugged Cat? I am leaning towards this one. A cat that just sat there rusting away. There is a small exhaust leak right before the cat that drained a bunch of water out when we first started it. My flaw in that theory is that it revs in park. Does the engine produce less volume or pressure of exhaust without a load?

Tranny seems good

Not burning oil.

No obvious signs of head gasket or other similar issue.

No obvious vacuum leaks.

No check engine lights. (Thinking a bad sensor would have tripped this)



Since the burb is in Anchorage and I am in Wasilla (60 miles) I am really trying to narrow down the problem to the most likely culprit as it means I have to drive in to fix it on the weekend(s) without my normal set of tools or my nice warm work space.

That and I hate blindly chasing gremlins. :Mechanic:

Thanks in advance, let er rip.:victory:

 

[Terrainist]

Start with the simplest things first. Every single vehicle that I've ever had problems with that sat -- bad gas. Always clogged the fuel filter. Might start with the fuel filter, and if there is any additive to strip out the gum from bad fuel in the system -- might try it. Never had to mess with cats, so can't say anything to that. The stink is indicative of something going on, wether bad burnt gas going thru a good cat, bad cat, or good gas striping out gum, etc.

And I always start to rationalize my way out of bad gas as well, "can't be cause' of this". It runs, it sputters, it stops running, then it runs fine, then... go the bad gas route. Maybe some good solvent ran with the gas will strip all the gummage out of the injectors, lines and pump as well? Hope this helped.

 

[BCHauler]

Hate to say it, but fuel pump is a likely cause. Enough pressure for no load but not enough when the engine has the load of the drivetrain on it. Check the fuel pressure before you drop that tank.

I had an 88 350 from a Burb in my Astro van and we had similar problems after it was swapped. Problem there turned out to be some bent prongs on the chip in the pcm.

When you say stinky, do you mean rich?

 

[Hill Bill E.]

Sat for 5 years....

Did you check for 'mouse houses'? They'll plug up an airbox in a heartbeat, also they'll get into the exhuast, and HVAC stuff.

I'd also second the gas, if any sat in the tank for that long, it's going to be gummy/watery/bad.


The revving fine in N, but not under load, sounds like either restricted air intake, or bad gas/fuel pressure.

Possibly a plugged cat, seen them do that. If it is that, the cat will glow cherry red where it is plugged, after a few minutes of running.

Be careful of that, my brother in law burned up his truck from a plugged cat.

 

[HenryJ]

...
Possibly a plugged cat, seen them do that.

My guess too. Pull the O2 sensors to relieve the back pressure and see if it runs better.
If your cat bolts in, take it loose and take a look.

My experience with a plugged cat:

On the return trip and I guess, a little noticeable while pulling the hills on the way out for the extended memorial day weekend, the intake noise started to noticeably increase while the power decreased.

The temperature did rise a little quicker than usual, spark knock over 4000 rpm began to get worse.

Today it would barely make 50 mph, bogging the whole time.

After checking all the easy stuff (ignition, fuel, etc) I talked myself into the fact that it must be the cat.

It was. The pre-cat was nearly completely plugged.

Here is a site that shows some of the damage and causes of converter failures-
Magnaflow website - converter failure

CATALYTIC CONVERTER FAILURES:
If a catalytic converter needs replacing, one of the problems below most likely
contributed to its failure.



Engine Tune-Up Required.
A number of problems could occur to the catalytic converter as the result of an engine that is out of tune. Any time an engine is operating outside proper specifications, unnecessary wear and damage may be caused to the catalytic converter as well as the engine itself. The damage is often the result of an incorrect air/fuel mixture, incorrect timing, or misfiring spark plugs. Any of these conditions could lead to catalytic converter failure or worse.



Excess Fuel Entering Exhaust.
The fuel that powers your vehicle is meant to burn in the combustion chamber only. Any fuel that leaves the combustion chamber unburned will enter the exhaust system and light-off when it reaches the catalytic converter. This can super-heat the converter far above normal operating conditions and cause a meltdown. Possible causes are an incorrect fuel mixture, incorrect timing, corroded spark plugs, a faulty oxygen sensor, sticking float, faulty fuel injector or a malfunctioning check valve.



Oil or Antifreeze Entering Exhaust.
Oil or antifreeze entering the exhaust system can block the air passages by creating a heavy carbon soot that coats the ceramic catalyst. These heavy carbon deposits create two problems. First, the carbon deposits prevent the catalytic converter from reducing harmful emission in the exhaust flow. And second, the carbon deposits clog the pores in the ceramic catalyst and block exhaust flow, increasing backpressure and causing heat and exhaust to back up into the engine compartment. Your engine may actually draw burnt exhaust gasses back into the combustion chamber and dilute the efficiency of the next burn cycle. The result is a loss of power and overheated engine components. Possible causes are worn piston rings, faulty valve seals, failed gaskets or warped engine components.



Deteriorated Spark Plug or Spark Plug Wires.
Spark plugs that don’t fire, or misfire, cause unburned fuel to enter the exhaust system. The unburned fuel ignites inside the converter and could result in a partial or complete meltdown of the ceramic catalyst. Spark plugs and spark plug wires should be checked regularly and replaced if damaged or if wires are worn or cracked.



Oxygen Sensor Not Functioning Properly.
An oxygen sensor failure can lead to incorrect readings of exhaust gasses. The faulty sensor can cause a too rich or too lean condition. Too rich and the catalyst can meltdown. Too lean and the converter is unable to convert the hydrocarbons into safe elements and may not pass a state inspection.



Road Damage or Broken Hangers.
The catalyst inside a catalytic converter is made from a lightweight, thin-walled ceramic. It is protected by a dense insulating mat. This mat holds the catalyst in place and provides moderate protection against damage. However, rock or road debris striking the converter, or improper or broken exhaust system support can cause a Catalyst Fracture. Once the ceramic catalyst is fractured, the broken pieces become loose and rattle around and break up into smaller pieces. Flow is interrupted and backpressure in the exhaust system increases. This leads to heat build-up and loss of power. Possible causes of a catalyst fracture are road debris striking the converter, loose or broken hangers, potholes or off-road driving.

A catalytic converter will rarely fail without a problem or malfunction occurring somewhere in the emission system in front of the converter. It is important to determine what caused the converter to fail so that the problem can be fixed and to prevent a recurrence of the failure.



Converter Meltdown.
This is an example of a converter meltdown. The converter was super-heated due to a raw fuel condition in the exhaust flow. The excess unburned fuel ignited when it struck the hot ceramic catalyst and drove the temperature far above the normal operating condition of the converter. The ceramic catalyst is unable to withstand the extremely high temperature and begins to melt. The ceramic collapses and the converter is destroyed. The melted ceramic may block the exhaust flow and cause additional damage to the engine. A converter glowing red-hot, or evidence of heat discoloration, confirms this situation.

The condition that led to this converter meltdown could be the result of a number of malfunctions including faulty oxygen sensor, an incorrect fuel mixture, worn spark plugs or plug wires, a faulty check valve, incorrect ignition timing, sticking float, faulty fuel injectors or other ignition malfunctions.



Carbon Deposits.
This is an example of a converter with carbon deposits in the ceramic catalyst. This is usually a result of oil or antifreeze entering the exhaust system or a too-rich fuel mixture. The heavy carbon deposit clogs the converter and reduces exhaust flow. This increases backpressure and causes the entire exhaust system to heat up. The heat backs up into the engine compartment and may result in a number of heat-related engine problems.

Mild carbon scoring may do less damage to engine components but it may seriously affect the converter’s ability to reduce harmful emissions. It could easily cause a vehicle to fail an emission test.

Carbon deposits may be the result of faulty valves, worn piston rings, worn or leaking gaskets or lead in the fuel.



Catalyst Fracture.
This is an example of a catalyst fracture. The ceramic became loose, cracked and began to break down. The pieces began to obstruct flow, creating backpressure and increasing the heat in the exhaust system. There is evidence of a partial meltdown in this example due to overheating.

The initial cause for this damage could have been road debris striking the converter based on evidence of impact on the converter shell. In some cases, if the protective mat that holds the catalyst in place is directly exposed to exhaust gasses, it could deteriorate and allow the catalyst to fracture. The Car Sound converter uses two recessed cavities in the body to hold the protective mat out of the exhaust flow to prevent any deterioration. The mat stays in place and the catalyst is held firm.

Catalytic Converter - By Larry Carley

C...
Driveability symptoms such as a drop in fuel economy, lack of high speed power, rough idle or stalling are classic symptoms of excessive backpressure due to a plugged converter. Checking exhaust backpressure and/or intake vacuum will tell you if there's a blockage...
To diagnose a plugged catalytic converter, you can check intake vacuum or exhaust backpressure. To check intake vacuum, connect a vacuum gauge to a vacuum port on the intake manifold. Start the engine and note the vacuum reading at idle. Then increase engine speed to about 2,500 rpm and hold steady. Normal vacuum at idle for most engines should be 18 to 22 inches Hg. When the engine speed is increased there should be a momentary drop in vacuum before it returns to within a couple of inches of the idle reading. If the vacuum reading is lower than normal and/or continues to drop as the engine runs, it probably indicates a buildup of backpressure in the exhaust. Remember, though, that intake vacuum can also be affected by retarded ignition timing and valve timing. What's more, some engines are much more sensitive to small changes in intake vacuum than others, so checking backpressure rather than intake vacuum may give you a better indication of what's going on...

 

[njtaco]

IIRC, there are small fuel screens in the injectors on that vintage TBI. Might be worth a look. If there are two injectors, are they "spraying" evenly under load?

Is the timing advancing/retarding as it should? I don't remember how the HEI in that vintage operates, but a stuck vacuum advance may contribute to your ills. The last time I pulled a Chevy out of a field, the air filter housing was FULL of nests, and the timing was 10+ degrees out of whack. After looking for the timing mark for a minute or so, I just started advancing until we found it. Poor 305 was doggier than normal trying to run like that! The previous owner thought that was just how they ran.

X3 or whatever on bad gas and rodent infestation.

How did the inside of the distributor cap look?

 

[matt s]

Wow lots here to reply too. Sadly you may all be right!

Still leaning towards the cat, but won't have a chance to play with it until next weekend. One thing it did, that I forgot to mention, is start backfiring when I started to loosen the aircleaner nut while running. I was trying to get a look at the tbi while running. Spooked me as the lid started popping up with the blow back. Tighten it back down and the backfire stops and it idles normal again. Weird.

No mice nests were found! We did check that one before firing it up the first time.

Current plan is to pull the cat and use a flex pipe just to test it out (I plan on replacing the exhaust anyway). Then from there go to the fuel filter. After that I will pressure test the fuel line, and if neccicary siphon and then drop the tank. No fun, but if that's what needs to be done I guess that's what needs to be done.

 

[Larry]

How did the inside of the distributor cap look?

Bingo… I would check the distributor cap first for moisture and corrosion. The small HEI caps have a habit of holding moisture causing issues like this. TBI engines are a real hassle to check fuel pressure unless you have the adapter that goes in between the TB and fuel line, which most people don’t so I would pull the cap first.

Probably more than one thing going on with this Sub after taking a 5 year hiatus.

 

[matt s]

Yeah I will pull the cap too and take a peek.

 

[shortbus4x4]

If its running bad under load but ok with no load and the smell is like its running rich I would check the ignition components first. You could have a component that is starting to fail, it will show up pretty quick when you put a load on it. Check the distributor cap and rotor, check the plug wires too, mice like to nibble on them. I would change the fuel filter. If your tuneup components look good which I doubt they will then check your fuel pressure. Pull the air cleaner off and check your injector spray too with the vehicle running, their is a slight chance your injectors are gummed up. You can use a timing light to get a good idea of if its pulsing correctly. I don't think you will find a problem with your injectors though. Checking fuel pressure on a TBI is a pain because you need a special adaptor but from the sounds of your description I would say its tuneup or fuel pressure related. Could be a plugged CAT but from my experience it would show up with no load on the engine too when you revved it up. You don't need to pull the CAT to check, just take the O2 sensor out, this will allow enough exhaust to escape for it to run. I have done this on a few vehicles that were towed in and would barely run, pull the O2 out and you can drive it to the exhaust shop. Just from your description I would start with tuneup stuff first because that is the easiest and it really sounds like you have a problem there.

 

[BCHauler]

X3 on the dist cap. Every sbc I've had of that vintage had problems with corroded contacts in the dist cap after sitting.

 

[Terrainist]

Just to reiterate, replace that fuel filter. Make sure it is one of the first things (should be first) you do on your get-it-running punch list. It will nullify other things you do if you don't replace it right away... been there. Quite probably you have multiple things going on, scratch that fuel filter off the list by replacing it. And stay on top of it. It most likely will continually gum up and get debris infested for awhile. Will totally make your fuel pump act goofy if it doesn't stop working altogether.

 

[matt s]

Thanks guys, fuel filter is on the list as well as popping the O2 sensor, checking the cap rotor etc. Will do one at a time to try and narrow things down.

 

[HenryJ]

Start with the easy stuff and work your way down. Tune up parts will be needed and are a good investment. Treat the gas and change the filter, clean the cap and rotor or replace as needed. Take a look at the plugs. Reading them can tell a story. Check the battery cables and connections. Make sure the battery is charged and the alternator working.
If you have good air, fuel and fire all that is left is to make sure the waste can get away. It all has to work together.