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problems with tbi 350 in 91 burban

moturbopar

moturbopar

1/2 ton status
Joined
Feb 9, 2005
Posts
387
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Location
mesa az
Hi all I traded for a 91 suburban thing, its suburban body cut down to fit on a short bed chevy truck frame. Anyways im having a problem with #8 cylinder on hte truck. The engine runs well except #8 is dead. I cannot find the problem to this. I have gone through and did a tune up New cap, rotor, wires and plugs, I have also rebuilt the tbi. New injectors, and a 14psi fpr spring. no matter what i do # 8 is dead, you can feel it through the exhaust, and when you pull # 8 plug wire there is no difference in the way it runs. I have checked for vacuum leaks and have found nothing. Checked compression its good at 157psi. did a leakdown on the cylinder was 8%, which seems ok to me. cam seems fine, no broken springs. Whats weird is the cylinder will somewhat fire when cold, but completely goes away when its warm. If I pour fuel into the tb #8 cylinder picks up untill the extra fuel is burned off. I have checked fuel pressure and it seems to vary from 9 to 12 psi at idle. I am leaning to it being fuel related, My next step is pulling the tank and replaceing the fuel pump. It sat for 4 years or so, so maybe its a dirty tank, clogged pickup screen? Thinking this as the filter had some rust in it when I changed it.
Anybody here have any ideas? Am i going the right direction? Im usually good at these trouble shooting problems, and dont like throwing parts at it to fix something, but this one has me doing just that!! Lol
 
Did you check the intake manifold? Maybe there's gunk or some kind of restriction in the fuel runner for that cylinder. :dunno:


If it sat for 4 years it might not be a bad idea to drop the tank and have a look-see in there to see what condition the rubber hose is in on the pickup assembly. Sometimes those thing swell and deteriorate and it causes a low pressure problem.
 
A problem with #8 has nothing to do with fuel. TBI has two injectors, at a minimum four or all eight are going to be affected if it's a fuel problem either with pressure or the injectors.

Sounds more like an ignition problem to me.

Did you do plugs before or after you found this issue? I'd suggest swapping the plug from that cylinder to another.

9-12PSI is in spec, but it should hold a constant pressure at a specific throttle/load condition, such as idling. If it's bouncing around, you've likely got a problem with the hose in the tank or the pump itself.
 
dyeager535 said:
A problem with #8 has nothing to do with fuel. TBI has two injectors, at a minimum four or all eight are going to be affected if it's a fuel problem either with pressure or the injectors.

Sounds more like an ignition problem to me.

Did you do plugs before or after you found this issue? I'd suggest swapping the plug from that cylinder to another.

9-12PSI is in spec, but it should hold a constant pressure at a specific throttle/load condition, such as idling. If it's bouncing around, you've likely got a problem with the hose in the tank or the pump itself.
Click to expand...


Thats what I thought also. I have tried switching wires, plugs, no difference! Maybe the pickup in the distributor? I cant figure out why it would pick up that cylinder when I pour fuel down it. this is one reason I was thinking a big vacuum leak, but I cant find one. pluged the vacuum t on that runner and made no difference. seems like if it was a bad gasket on hte inside of the engine it would suck oil and smoke, but it doesnt.
 
If you have a known good distributor, it would not hurt to swap it in. These things are of the age that they start exhibiting some strange issues as the distributor components go, it's cheaper now to buy a complete replacement distributor than it is to buy all the parts. And to replace the pickup coil the distributor has to come out anyway, so less work to buy a complete new one.

That's *if* you can justify a new distributor, whether as a spare, or because you want to make sure* you have a good distributor.

*parts made and/or assembled in China should never be considered problem-free even when new.
 
dyeager535 said:
If you have a known good distributor, it would not hurt to swap it in. These things are of the age that they start exhibiting some strange issues as the distributor components go, it's cheaper now to buy a complete replacement distributor than it is to buy all the parts. And to replace the pickup coil the distributor has to come out anyway, so less work to buy a complete new one.

That's *if* you can justify a new distributor, whether as a spare, or because you want to make sure* you have a good distributor.

*parts made and/or assembled in China should never be considered problem-free even when new.
Click to expand...
Actually I just purchased a new one. I was reading that the magnet can crack and cause weird problems. will probably swap it out tomorrow.
 
Have you pulled the valve cover and looked over the rockers ect to see if you've got a sticking valve? done compression check?

Take a look down #8 intake runner see if theres something sucked in like a rag..

Put a timing light on #8 see if it fires the timing light
Try a spark checker on it and see if you get an arc.


You ever make it out to the west side stop in I'll look it over with you.

Josh
 
JoshHefnerX said:
Have you pulled the valve cover and looked over the rockers ect to see if you've got a sticking valve? done compression check?

Take a look down #8 intake runner see if theres something sucked in like a rag..

Put a timing light on #8 see if it fires the timing light
Try a spark checker on it and see if you get an arc.


You ever make it out to the west side stop in I'll look it over with you.

Josh
Click to expand...


Yep, valvetrain looks fine. no sticking valves, cam is good, no broken springs. in fact the engine is very clean. leading me to think its was very well taken care of, or low miles.

Plug is firing, checked with an old style spark checker.

Yes compression is 157 psi on that cylinder, with 8% leakdown.
As for something stuck in the intake I dont think there is. When I did the leak down, I opened the intake valve while I hooked up an air hose and air came out the intake.

Thanks for the offer, i might need it!!
 
how does that plug "read" compared to the others after some run time? are you sure it's actually firing?

check your spark by putting an old plug in the hole, put a plug in the plug wire, ground it and start the rig.... can be a big difference between what you see with some spark checkers, coughscrewdrivercough, and what the actual plug is firing as.. all spark is not created equal...
 
I've seen several chevy small blocks that wont run right with platinum plugs,especially Bosch ones...
For whatever reason one out of the 8 (or more) simply refuse to fire in a particular cylinder...my friend ran into this many times and after replacing the coil and checking the fuel delivery to that cylinder,he'd put one of the old plugs back in and ta-da , the engine no longer had the "dead cylinder"...
I cant say why,he has no clue either..:screwy:..

He'll only use AC "regular" spark plugs in GM's now..if a customer brings his own parts and he bought platinum plugs,he gives him the choice of returning them and swapping them for AC's,or he'll tell them "if it skips with the new plugs,it aint my fault,and I don't want to be pulling them back out for free"..
 
diesel4me said:
I've seen several chevy small blocks that wont run right with platinum plugs,especially Bosch ones...
For whatever reason one out of the 8 (or more) simply refuse to fire in a particular cylinder...my friend ran into this many times and after replacing the coil and checking the fuel delivery to that cylinder,he'd put one of the old plugs back in and ta-da , the engine no longer had the "dead cylinder"...
I cant say why,he has no clue either..:screwy:..

He'll only use AC "regular" spark plugs in GM's now..if a customer brings his own parts and he bought platinum plugs,he gives him the choice of returning them and swapping them for AC's,or he'll tell them "if it skips with the new plugs,it aint my fault,and I don't want to be pulling them back out for free"..
Click to expand...

Im running the plain old AC delco plugs. Changed them around and no difference. Today fuel pressure is barely hitting 8 psi, so Im going to do the fuel pump and see what happens.
 
Not that they are made in the US anymore, but I'd highly recommend a Delco or one of the other pumps that comes highly recommended (not sure if it's Walbro or who) unless you enjoy pulling your tank every year or so.
 
I have had very good luck with the delphi pumps.

As far as this miss, only at idle? Under load?
 
black dawg said:
I have had very good luck with the delphi pumps.

As far as this miss, only at idle? Under load?
Click to expand...

i purchased the ac delco ep241. its for a tpi equiped vehicle, my plans call for a multi port EFI cadillac 472 at some point.


The miss is at idle and under load. re did the compression test and it still read 157psi.

I took a syringe with fuel in it. and hooked it to the vacuum T on # 8 cylinder. everytime a pushed fuel in # 8 would pick up.
when i checked the plug after 10 minutes of it idling it is still perfect white. With the fuel being pushed in it actually had a tan tint to it. new distributor is ready to go in and fuel pump showed up. so I know what im doing this weekend!!
 
Have you made sure none of the vac hoses are leaking on that port. I have seen many idle misfires on 8 (on tbi stuff) because of leaky heater controls/actuators.
 
JoshHefnerX said:
Did you get anywhere on it this weekend. Was pretty hot out...
Click to expand...

Got the tank out. Its clean inside, but the sending unit is very rusty and so is the pump (cheap airtex pump). I am waiting on a new sending unit to come in, should be here today.
 
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