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Truck broken down, need ideas

Blue85

Blue85

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I'm sitting at work trying to get my truck to run. I just barely made it here earlier. I have a new MPFI system which may or may not be to blame. On the bright side, I can monitor all of the sensors and signals with a laptop.

The truck started feeling sluggish and just got less and less power until it would barely run and I couldn't even get it into 2nd gear. O2 gauge read pig rich. The engine vacuum is so low that I thought I had a plugged catalytic converter (or 2). Plus, I could have sworn that as I was limping it along I heard an unusual whooshing sound whenever I gave it any throttle. I unbolted the exhaust from the headers and it didn't make much difference (except for the sound). When I start it, it has like 15" of vacuum, but this fades down to maybe 5" over about 20 seconds before it dies. Fuel pressure is good the whole time. It also seems to be leaking a lot of oil, but I don't know if this is related or not. Timing reads correct with the timing light.

Here are my ideas:
1) bad spark. It is sparking, but I guess I can check the plugs for damage and make sure there is current on all 8 wires.
2) Leaking or dead injector: Hard to tell for sure because my fuel pressure gauge is leaking a little. At any rate, it doesn't leak enough to bring down the rail pressure. If it was leaking and that was my only problem, the engine should run better as load increases.
3) slipped timing chain. Maybe it jumped a tooth causing the lack of power and then jumped another making things really bad. I guess I could pull the rocker covers, but checking all 16V might take a while.
4) Cam/lifter/rocker gone bad.

Any ideas are welcome. I would like to get it out of this parking lot tonight and back home.
 
When my intake stud broke, I had similar symptoms, except it smoked like crazy after running a few minutes (sucking oil past the rings) and I don't recall it really stalling on me. No vacuum reading, but it shouldn't have been low like yours. It was sluggish, but not as bad as you describe.

I've seen exactly one GM that the timing chain slipped, and it was very high mileage. IMO there can be no "single tooth slip", either it slips, or it doesn't. When it slips it's because the nylon is shot, and once it's gone enough to slip, nothing is going to keep it from continuing to slip.

Whats going to cause low vacuum? Certainly not bad spark.
 
But initially high and lowering as it runs? That seems odd to me.

Where is vacuum being measured, is it straight from the MAP sensor?
 
It's the slowly shrinking vacuum that had me looking at the cats. I can check for intake leaks with my stethoscope. There is no oil from the tailpipes, no sign of water in the oil and no signs of oil in the coolant. The injectors are not leaking because I fixed the pressure gauge and it holds pressure for a long time. It seems like an intake leak would make it run lean, but it is running really rich, even when I disable O2 feedback.

I managed to get the truck inside my loading dock, but I had to put it in low range to get up the ramp into the building without stalling. I installed a hoist and exhaust vent in here, but have never used them for work-purposes. At least all the tools are getting used now :-)
 
dyeager535 said:
But initially high and lowering as it runs? That seems odd to me.

Where is vacuum being measured, is it straight from the MAP sensor?
Click to expand...
I have a line coming from the Plenum which tees off to the MAP sensor and my A-pillar vacuum gauge. The gauge reading match those on the laptop. I have been using both for a while and have no reason to suspect them.
 
I'm having trouble finding any vacuum leaks. It runs so rough that there is a lot of noise. Plus, the exhaust is disconnected again.

I ran it the headers unbolted from the pipes again and it stays running for a minute or more. The vacuum sits at 18". It still dies whenever I do anything. This morning the vacuum was 20.5" at idle.
 
The PCV is sucking. With it pulled, the engine stalls, but if I cover it with my finger, it runs the same as when it is plugged in.

I have found a real problem, but I'm not sure if it tells the whole story. Apparently I am intermittently losing spark. When this happens, it locks up the ECU and I have to cycle the key. This doesn't explain "lack of power", but I'll have to address it. It happens when I wiggle the wires to the distributor. Maybe the ground is intermittent and a spike goes back to the ECU.
 
Oh yeah, I have been running it with the exhaust disconnected and getting 20" of vacuum. I'm not sure what has changed. It idles indefinately until I mess with the dizzy.

I have been all over the intake, runners and plenum with the stethoscope now and I can't find any leaks.
 
IIRC............vacuum drops as you open the throttle..............maybe not getting enough fuel.
 
I had a broken lower injector o-ring once. It would run really rich, as much as blowing strait carbon out the tailpipe, and a lot of black smoke. Barely any useable power. It would run fair when it was cold, but when the engine was up to operating temp, it would drive terrible.

I tested things for days and couldnt find it. I used my timing light to show me the injector patterns, and thats when I saw the fuel droplets. That was with TBI though.

With the TBI, the fuel cavity fills up with gas, then the injuector sucks the gas from that pool and shoots it into the intake. Your MPFI doesnt work that way, does it?

Ive been trying to think of what can cause that problem, but Im at a loss with your current symptoms.

Just came to mind... My friends father had a s10 Blazer one time acting real bad. Ended up being a bad mechanical fuel pump. It was making a certain harmonic that was being picked up by the knock sensor, and in tuen, would retard the timing. Maybe look at your knock counts???
 
Knock detection is part of my future plans...

The TPI system has an injector just in front of each intake valve. The only way to physically inspect the spray is to remove the intake from the engine, or take the injectors/fuel rails off of the intake, so there's no way to look at it while the engine is running.

Now why can't poor spark cause low vacuum? Can't misfires make drops in the vacuum? Let's say I start the engine with weak but present spark and as the coil (or something) heats up, the spark gets worse and worse. Can't that cause more and more misfires and worse and worse vacuum until it isn't running at all?

I did find a poor ground to the ignition coil and I fixed it. Now the intermittent lock-ups are gone. M_A_Y_B_E it was a coincidence that things improved when I unbolted the header collectors. I had been hooking up an O-scope at the dizzy.

I am starting to feel some comfort that it is not an internal engine problem. I might try driving it.
 
Underneath the truck, the floor is covered with little black grains of what I assume is carbon. If it's been doing this all day, then the cats probably are plugged. It has to be super rich to do this, doesn't it? Maybe my wideband O2 sensor is calibrated wrong. With the exhaust disconnected it measures way lean, but that is because it is mounted in the header collector, so it's like 1" from being in free air.

Otherwise, it seems to idle good in in Park and in gear.
 
I hooked the exhaust back up and it runs really bad again. The vacuum is again dropping the longer the truck runs. I put a bunch of silicone on as makeshift gaskets and the pressure blew it out and starting making a wheezing noise.

So I am pumping out carbon. Why and how do I fix it?

I'm heading home. Thanks for all the replies. Hopefully I'll figure it out tomorrow after I get some gaskets and cut out the cats.
 
Well, the IAC is a controlled air leak, if the ECM is letting in as much as possible through the IAC, I can see that the vacuum would go down, especially since it's stalling out. Typically it would all balance out, but if it's not compensating for the increase in airflow, then stalling and vacuum loss might be noticeable.

Another (weird but understandable now) symptom when my rocker stud broke was that the runner filled up with fuel. No way the ECM knows the intake wasn't opening, that fuel was spilling out of the runner into the plenum once it filled, and I finally got a "rich O2" code.

However, if you are still seeing problems with the exhaust hooked up, and signs of pressure building there, your plan to "open" the exhaust is a good one. If you've got the ability to pressure test, you've got the O2 sensor fitting to use. Don't need a known problem contributing symptoms on top ofan underlying issue.
 
I'm almost sure now that the low vacuum is due to plugged exhaust. The pressure builds up behind the exhaust valve so that the combustion chamber doesn't empty on the exhaust stroke. When the intake stroke happens, the chamber is already full, so it doesn't suck much air in and intake manifold vacuum is weak.

So I'm focusing on the question: why is my engine making carbon?

The IAC is acting the same as usual. The engine was never cooling down so it basically opens all the way during crank and then closes all the way a few seconds later. I configured all this and I can see the IAC steps on the laptop.
 
Is it simply that the tune is too rich?

You've got the simple stuff, wiring bad somewhere? (cut, burned, etc) Plugs, wires all good?

My truck will run on 4 cylinders (one bank of injectors connected) and on 7 cylinders, its almost impossible to tell by idle quality.
 
how about..

Is the EGR valve still in operation?...and is it opening at idle when it shouldn't be?..that can cause manifold vacuum to plummet!..so can plugged catalitic converters..sometimes they only plug up when they get hot...:crazy:
 
This seems ridiculous. I must have about a cup of this carbon on the floor now. Both cats are completely plugged with it, of course. Can the engine really produce this or have I sucked it out of my charcoal canister?
IMG_2557.jpg


(never mind the oil, it was there before).

The EGR should work, but I actually had the vacuum signal to it disconnected for the last week or so. I know the passages to it are open, but I never really verified whether it is flowing air or not.
 
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