Suburban help

I decided to start a new topic instead of adding to the OMG a tow vehicle since it will be easier to follow.

I have a 2004 suburban, 1500 with a 5.3 vortex. The issue is I'm lacking power and can't give it more than 1/4 of throttle or it will run like shit.

My mechanic thought it was dirty injectors at first, we put a bottle of cleaner and it was a lot better. Before that it would completely die once in a while at idle. Never did it again after that but I was still not able to give it more than half throttle. I also cleaned the MAF and TB.

It gave a code for the 2 first O2 sensors, got that changed, did nothing I could see or feel.

Gen said it could be the fuel filter, told that to my mechanic and he was not sure. He told me he changed it but I'm not sure. He's an old Italian and you know when they don't want to do something, they won't.

But I think that the fuel is ok. If I rev it in neutral it will miss and I can hear some backfires in the cat. I tought it might be some bad plugs.

We looked at the plugs and it looked like they were the ones the truck came with new... it now has close to 200k kms(120k miles). So I changed 6 of them over the weekend. Why only 6? Because I have no idea how I can change the one closest to the firewall on the passenger side. If I get my ratchet on it, I'm at an angle where it won't turn. And yes I'm using an extension and a joint (?).

The other one I did not change is also the closest to the firewall but it was only because I ran out of time.

Now it is idling rough, almost as if one cylinder is missing, but not as bad. If I unplug one of the plug wire, I can see the difference.

Oh and in neutral, if I don't give more than 1/4 of throttle I can bring the RPM to about 5000, but as soon as I give more throttle, I can't get past 3k and it will miss and backfire.

So 2 questions now. How the fuck can I get to that spark plug on the passenger side? (my mechanic is not there till Friday so I can't have him do it now).

and Can it be the 2 old plugs combined to the 6 new ones that gave the rough idle I didn't have before?

Oh and a third one, wtf can make my truck run like that.

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Francois

Have you checked fuel pressure?

Have you checked manifold vacuum, at idle, while free reving at about 2500, and while power braking?

Its low mileage, but I would check mechanical timing, make sure the chain didnt jump.

For the spark plug, does your spark plug socket have the nut on the back of it? Try putting the socket on, then turning it with a wrench to get it loose.

Sounds likje a classic case of restricted exhaust, or low fuel pump volume. Put a fuel pressure gauge on it, DRIVE it and make sure the fuel pressure does not drop off at WOT.

If you don't have access to a back pressure tester, loosen the pipe connections at the headers and drive it.

We've never had an issue changing plugs, so I need to go look at mine here shortly and see why you're in trouble.

Those engines are about as problem free as they get (other than oil leaks and the Castec head issue). Misfire could be a bad plug, even new ones have problems, bad plug wire, shitty fuel, a failing MAF can cause this also. Cleaning doesn't fix a faulty one. Vacuum leak somewhere. Temp sensor.

Should be misfire codes, throw a scanner on it and see what you have. At this stage I'd suggest a new mechanic.

Get a fuel pressure gage on it and go drive it, see what you have. Chevy gets their fuel pumps out of Cracker Jack boxes...

Check back pressure and see if the cats are failing. Or do a temp check after driving. Should be 100* hotter at the rear than the front. If plugging will be oppositish.

I think the best advice is to get a new mechanic.

This one is good at changing parts, but troubleshooting is not his cup of tea.

I'll give a call to my friends who prepared my Laser rally car, he's now running a subaru shop but he had a few Yukon and Suburban. I trust him more than the Italian guy anyways, it's just that he's about an hour away from me.

As for the plugs, maybe I just don't have good enough tools, or I just don't know how to use them! But I do think Ian is on the right track with the wrench on the nut of the socket! That should work.

I don't have any back pressure tester, fuel pressure tester or even a way (other than my hand) to compare exhaust temp, but I'm quite sure my friend have all that at his shop. Well maybe not the back pressure tester, but that's easy enough to test without it by unbolting the exhaust.

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Francois

I just looked at mine..we use a wobbly headed Snapon ratchet. makes the plugs easy to get to.

Fuel pressure gages are pretty cheap at the local parts store. Probably ought to have one anyway. Backpressure gages are easy. Just find or make a bung with a nipple (insert Beavis and Butthead snickering here) that fits in the O2 hole (more snickering). A normal old vacuum gage usually has a pressure scale also. Anything over a couple pounds at idle is not good. Nothing should show over 4-5 lbs under a hard load.

Well I just gave a call to my friend and he'll be staying at the shop Thursday night so we can have a look at it.

He have all these fancy tools and gauges so it will be easier to find what's wrong.

Is it possible that an O2 Sensor and a compression gauge could have the same thread? Would make an easy back pressure gauge! Or is that a bad idea? Since it's the one that stays at the highest pressure, I could even go for a test drive while it's in!

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Francois

Its the fuel filter. It's tiny and needs constant changing.

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Owner, Driver, Head Mechanic, and Janitor at Tower City Race Team, headquartered near L'Anse. Michigan.
Driver / CoDriver in Rally America, NASARallySport, ARA, AMS, UPMDA, Champ Off Road

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Injector replacement was common at the dealership level, too.

A word of advice. If it throws "O2 codes" it is usually NOT the O2 sensors. If you can cause vacuum leaks and make the O2 read lean, and add propane and make the O2 read rich, look elsewhere.

Also, don't use Bosch O2 sensors. A lot of them are bad right out of the box, just like their spark plugs.



Edited 1 time(s). Last edit at 10/25/2011 07:09PM by Pete.

Quote
Francois


Is it possible that an O2 Sensor and a compression gauge could have the same thread? Would make an easy back pressure gauge! Or is that a bad idea? Since it's the one that stays at the highest pressure, I could even go for a test drive while it's in!

I have seen kits that come with o2 plugs, but the gauges have quite a high scale, and you are only looking for a couple PSI.

A vacuum gauge on the intake manifold will show restriction. Engine should read smooth 18ish in/hg at idle. When free revved, vacuum should dip to ten or so, then rise back to in between the two. If it sinks below 10 and stays just free revving, you have a serious restriction. Power braking you should see only a couple in/hg, but if it gets close to ambient pressure, or even spikes into pressure, you have a restriction.

If you have a non smooth vacuum reading at idle, you may have a chipped valve, or low compression. A leak down test would be advisable. Even with a no spark misfire, the vacuum reading should still be mostly smooth.

I think this new mechanic should be able to get you sorted out. Good luck, and make sure to update once the issue is solved.

Quote
Pete
Injector replacement was common at the dealership level, too.

A word of advice. If it throws "O2 codes" it is usually NOT the O2 sensors. If you can cause vacuum leaks and make the O2 read lean, and add propane and make the O2 read rich, look elsewhere.

Also, don't use Bosch O2 sensors. A lot of them are bad right out of the box, just like their spark plugs.

Funny how many times I see O2 replaced like this. Usually by the customer with the help of the $10hr parts counterman at Shucks.

Bosch, we've noticed the same thing. Bosch used to be a good name many years ago, now I get skeptical every time I see it. Really makes you want to buy a washing machine with their name on it.

yeah I was not so sure about the O2 too, but with overs 100k miles on them I don't think it was all wasted... and I only got charged for the parts so not too bad.

A friend came with his brand new wobbly headed ratchet and took care of that plug. Now it's back to square one as it's now acting exactly like before I started to change the plugs. Idle is fine, part throttle is fine, WOT not so good.

One of the plug is full of carbon, so I think that's what was giving me the rough idle since Monday.

Tomorrow night I will bring it to my friend and see what's causing the WOT issue.

I think I'll buy some new plug wires too since they too have over 100k miles.

I'll keep you posted, but so far it's looking better every day! Maybe I won't have to set it on fire!

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Francois

Just to keep you up to date.

It's been over a week since I changed the plugs and cleared the misfire codes, still no check engine. I guess for once the O2 Sensor codes were in fact caused by the O2 sensors since they are not back.

My friend was quite sure it was a defective MAF, well he was wrong. I just tested it with a coworker's one and still the same issue.

I thinks it's time to buy a fuel pressure gauge and see how the fuel pressure is when it's doing it. I'm really not sure it's fuel related though since I can smell fuel and hear some backfires in the exhaust when it's having the issue.

What's bugging me is that it if I don't give too much throttle and not towing it rides like a brand new truck. Smooth running, rpms are climbing without any hesitations, nice idle, etc. But floor it and it starts running like shit.

Is there a TPS on that?

I think I have an old boost gauge from my rally laser. I'll try to hook it up to see what kind of in/hg I get...

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Francois

You could also have a coil breaking down under load. You should be able to watch the misfires on a good scanner. If you can and number 4(example) is the culprit then swap the coils one spot and see if the misfire moves.

wouldn't that cause the check engine light to come up?

I thought it could be that but since I changed the plugs it doesn't show any missfire codes.

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Francois