Hey guys, new here. I have a Cheyanne but all of the good TBI info I find is on here so I hope you can accept my pickup!

Long story short, I picked up a really nice '94 Cheyanne for my brother in law. It had 200k on it so I rebuilt the engine. It's a 305 TBI all stock except the cam which I upgraded to a roller cam from a '96 vortec engine. This thing has been nothing but a headache since.

The distributor looks pretty good inside and out so I reused it. First thing I did was set the timing-unplug the ESC tan/black wire, set to 0, reset, plug it back in. When it's operating under the computer again, timing kicks out to 12 degrees advanced.

The truck ran ok here at first. Idle seemed a bit low and it would drop really low when put into gear. I took it to the gas station and by the time I got back I had to neutral drop the thing and keep on the gas just to keep it running. I also had just about no power and a 33 code. It wouldn't start unless I gave it some gas to get going then it would idle really really rough at like 500rpm. I got a vac reading from the back port going to the map sensor. 10 in vacuum. Pcv valve seems to be fine, it's not too dirty at all. I can blow into it from the TBI side and get a little bit of air through, but it's not wide open. No visible leaks or audible hissing anywhere. Once I get the rpms up above 1500-2k it settles and sounds normal/revs normal. I checked over a number of things and can't seem to find what's going on.

Now here's the kicker. When I unhook the line going to the brake booster the vacuum stays the same, but the engine will smooth out and idle about 1k, but it's slightly hunting. If I plug the tube with my thumb, vac will go up, but I have no number as it will also cause the motor to stall. This part makes no sense to me.

Also the vacuum will fluctuate about 2in as I'm reading it.

Any help would be appreciated. Thanks in advance!

 

1. What is the cranking compression pressure of all eight cylinders? Cranking compression pressure is a half-assed (but easy) way to verify cam timing. I'm wondering if the camshaft is not properly timed.

2. Opening the hose from the vacuum booster makes the idle smooth-out? Seems like the air-fuel mixture is too rich.

3. Assure that the EGR valve is closed and sealing at idle. A stuck-open EGR valve will make for a terrible idle, stalling, etc., but can run "ok" at higher speed.

4. Connect a scan tool, find out what the computer wants you to know. Check for "codes", but also check the data stream. Verify EVERY sensor, and computer output. Look at the O2 sensor cross-counts (voltage swing). O2 sensors get lazy with age and mileage. How old is the O2 sensor? Make sure the IAC is working properly to control idle speed. Examine the fuel trim numbers. If the computer is in rich-command, that's a sign of low fuel pressure. How old is the fuel filter? What is the fuel pressure?

5. Did you examine the magnet on the distributor shaft? Common TBI distributor problem--cracked magnets. Makes for erratic timing, misfire, stalling.

 

Thank you. Very good info.

I'm going to start with the EGR and comp test after work tonight.

After thinking about it, it seems that the engine is pulling a vacuum below the throttle body. The map only sees small vacuum up top in the TBI and Richens the AF ratio. Does that make sense?

 

Totally normal and expected.
No. The vacuum passage, nipple, and hose leading to the MAP sensor should have exactly the same vacuum as the intake manifold. If there's a difference, there blockage, and/or leakage in the vacuum path to the MAP sensor.
OR
The MAP sensor is connected to the wrong vacuum nipple. Should be connected to manifold vacuum, maybe is connected to ported (timed) vacuum.

 

So essentially, if my EGR is hanging open I have opened another source to draw vacuum, hence lowering the vacuum pressure in the system. That would normally cause a higher idle since more air is being brought in and leaning out the mixture. Since it's exhaust gasses re-entering the system through the EGR then I'm actually richening the system by increasing the ratio of exhaust gas to intake air ratio, which causes the stumbling idle. When I take the hose off of the brake booster, I'm allowing more air into the system, leaning out the mixture and causing the idle to increase. Am I framing this correctly?

 

It's more complex than that because the O2 sensor will be reporting rich/lean to the computer, which will adjust the fuel delivery to compensate.

Point is, EGR leakage or improper activation makes for shitty idle on gasoline engines. Worth checking.
Faulty MAP sensor vacuum makes for improper base fuel delivery. Needs to be fixed.

 

I'll dig in tonight.

I checked vacuum at a few places a few different ways and it was the same everywhere I checked.

 

Which is it? The MAP is not seeing full vacuum, or the vacuum is the same everywhere?

 

Did not check vacuum anywhere before the TBI. That's what I mean. Checked it a few places at the TBI. PCV, EGR Solenoid port, map sensor port. Also used checked these places with different hoses.

Only places I can see to check the vacuum below the TBI are the EGR port which is covered by the EGR and the brake booster which stalls the engine when plugged.

 

Also I'm speculating/asking if it makes sense for there to be more vacuum under the TBI. Trying to understand the system so that I can diagnose it instead of just asking what parts I should replace.

 

First thing I did when I got home was check the EGR. Brand new out of the box as of last Friday. Wide open. Blew right through it. So I built a tamp blockoff plate and installed. Problem solved. Next thing I'm going to do is order a new AC Delco. Thanks for all the info and help!

 

Make sure the Delco one is a Genuine one with a tiny metering orifice on the vacuum tube end. If it is a full diameter vacuum tube it is going to cause you hesitation and bogging.

Also you need to advance the base timing a bit. Even a stock engine likes a bit more timing. I would a start at 4°, then move to 6°, then 8° and maybe as much as 10°. Before I was tuning the chips in these, I never found one that did not like atleast 6° BTDC. The factory timing map is beyond lame on the TBIs and the Vortec cam reduces the dynamic compression ratio a good bit compared to the stock L03 cam.

 

Ok, so I am asking just so that I understand how the computer advance works. I'm currently idling at 10* advanced with the wire plugged in, even though I set base timing with the wire unplugged at 0 degrees. What difference will it make if my base timing is set advanced?

 

It increases the whole timing curve if you will by the amount you change the initial timing. The TBI is lucky to have 18-20° of WOT timing in stock form. The TBI heads like 26-30° total timing. If you get the BCC off the Memcal in the computer I can likely find a copy of it and post the timing tables here. My 350 van was BJYN for example and it only had ~20° @ WOT stock, I ran that one at 8° advanced for a long time. After advancing the timing, it had better throttle response, more torque, better mileage and ran cooler. Give the engine what it wants to run well, not some some specification printed on paper that is no longer accurate because the engine is no longer stock.

If you have a loptop you can also get a ALDL to USB cable relatively inexpensively, download TunerProRT and download the definition files that match your PCM on Gearhead-EFI. Then you can datalog things like knock retard, fuel trims, commanded timing, etc.

 

When I get the new EGR put on in place of the block off I'll see how much total timing advance I'm getting. I'll probably adjust it accordingly. I appreciate the info!

 

The total you see with the EST connected includes the vacuum advance. Probably want to see 38-42° @ 3,000 with no load. When the MAP drops off under load it will retard the timing 10-14° or so from what you see with no load.

Some people would say to tune them and while I agree, I managed to get them to run very well with base timing and fuel pressure changes prior to tuning the chips back in the day. I had a LE9 305 carb engine with a RV cam and headers under a 350 TBI system. It was up from 165 hp at the crank to 191 hp at the tires with no changes to the chip.