1991 4x4 1500 !HELP!

Can somebody please assist me?!! My suburban has been steadily getting worse, and today I think I tracked the problem to the fact it's getting to much fuel. It has the TBI 350 in it with 250K on the engine. The symptoms are erratic; any thing from stalling on deceleration, such as if it was starving for air, to the engine "loping" when you barely touched the gas. It seems to run fine on the freeway, and it seemed to run better at higher elevations. It gets HORRIBLE gas mileage down here in 'Vegas, and is getting worse...I've driven this truck for three years so I know that this is HORRIBLE mileage! Anyhow, I've done the usual tune up type stuff, plugs, wires, cap, rotor, fuel filter, fuel system cleaner, and air filter. WHATS NEXT? As I mentioned, it seems to be drowning its self. The only way I can get it to duplicate the symptoms in my garage, is by putting it in drive, holding the brake, and cracking the throttle. When I did this, I can see drops of fuel splashing up. The temperature gauge tells me it's running at about 190 and today it started shifting into a higher gear when the engine would lope...until it reached operating temps...not normal...What else can be checked? I don't have any lights on, such as the check/service engine lights, and everybody I've talked to around here tells me that I can't have sensors out because that light isn't on/flashing, and yes, the bulb that's in it is good!! Is it not enough spark to burn the fuel? I installed a little hotter plug last week, no change. PLEASE HELP!
THANKS EVERYONE!

So you are saying the 2 injectors are driping fuel and not a nice cone shaped spray???
If so there is your problem

So you are saying the 2 injectors are driping fuel and not a nice cone shaped spray???
If so there is your problem


No, I think the injectors are acting properly, but is there a way to test them?

Also, I jumpered the pins on the ALDL port and read the codes today, I had a 33, 42, and a 43. The first one seems to correspond with the MAP sensor, which checked out fine. I checked the EGR valve by attaching a spare piece of vacuum hose to it and sucking on the hose while the engine was running, and the engine responded. SO..I had them check the EGR vlv solenoid, it checked out fine...SO...I took the ground off the battery, thus clearing the codes, and drove it home, about 6 miles, about 15 minutes worth. It seemed to be acting quite a bit better, except for when I made a hard left turn, it did it's "loping, running out of fuel, not enough air" thing. I'm thinking fuel pump? What pressure, and where to check it? Also, when I got home, no codes except the normal 12 code.........Fuel pump and injectors, which should be tested first? This seems to be an intermitent problem, but it's getting worse....AGHH!

If you can replace the MAP with a known good unit.
A bad map will act like what you have here.
Also a stuck open egr valve could also cause these problems.

If you can replace the MAP with a known good unit.
A bad map will act like what you have here.
Also a stuck open egr valve could also cause these problems.


I had the MAP tested at AutoZone, the guy told me it was good, so hopefully this means that it is.
How can one tell if the EGR vlv is stuck open? Last night I routed the intake manifold vacuum line directly to the EGR and it killed the engine. I could see the vlv stem moving when I was sucking on the vacuum line, trying to test to see if it changed anything on the engine.

I dont trust those testers very much.
I go to a junkyard and buy test sensors from running engines that way I have good test sensors.
Running rich, stalling, engine lope, you have done the basics with a GOOD tune up. A fuel pressure problem on that year no.
All the things you say sound like a bad MAP. Or bad ecm.
Here is what I want you to do.
Let the engine idle for about 20 min and dont touch the throttle. Shut down the engine and let it cool off. Now pull each plug and see if any are black or very white in color.
If you find any that are bad see if they are all on the same side of the engine.
After all that is done write down what you find. Now for the last thing with every thing back in place start up the engine and see if you can get the engine to act up if you can I want you to tap on the ecm and see if the idle changes if so replace the ecm if no changes go to the throttle body and disconnect 1 injector and see if idle smooths out if not reconnect and try the other 1
post what you find

Thank you! It'll have to wait 'till tomorrow, duty calls. I have had the time to got to the library, looked in a Chiltons Fuel Injection manual for code 33....causes are low vacuum and high voltage from MAP...Autozone tested mine with over 2.5 volt change....I'm thinking bad MAP....Tomorrow I will do exactly what you've said, as well as check the timing. Once again, THANKS!

1 other thing to check is at the base of the TBI unit is a gasket and is known to go bad and suck in a lot of air I would look real close at that also. With the air cleaner on and all vacuum line connected spray some carb cleaner around the base of the TBI unit and see if the rpms change..
Also check all vacuum lines for any leaks

Well, I've had the time to check the timing...Good.
I've also checked the vacuum lines....Good.
Checked the gasket....Good.
Haven't had time to check the plugs as you asked, but I did pull the injectors, one by one. The engine will slow down in RPM's, slightly hesitate, but come right back. This is the same for both of them.

Also, I don't know if this is a separate problem, but lately it's been shifting to the highest gear possible, even though the vehicle speed is no where near what it should be.......Think this is part of the same problem, maybe a bad ECM, getting worse? BTW, this afternoon it drove almost like when I bought it, which is to say it drove better this afternoon than it has in the last month.

Two more suggestions to check:

1) Check the TPS. Readings should be .5 volt idle and 4.5 full throttle. Nice and smooth in between. My 91 454 Burb had brutal shifts. Replacement corrected the problem. If voltage readings are inaccurate, you can have erratic shifts, and driveability problems.

2) Make sure the distributor shaft isn't worn, causing it to "flop" around. Mine was causing an engine flutter?, and when just crawling had intermittent lope. Would only do it occassionaly.

Did you find and fix the problem???????????????

I have a 89 Sub, and it sounds like i'm having the same problems you are, loping, bad gas mileage, lack of power, hard throttle(only when it acts up), no codes, and all intermittently. Some days it seems to run like i've injected nitro down it's throat, then other days it seems to be pulling an anchor! I have no hair left from this two year frustration. It's been in and out of the mechanics with the same result-no hair and no ideas.

I've replaced sensors, upluged sensors, changed prom and cal chips, even replaced ECM with all results the same- runs good for a few miles or a day, then back to it's ways.

Mine seems to idle smooth for a few seconds when i start it up for the first time after sitting for a day or so then goes in to a surge of RPM's (up then down then up and back down) untill it dies. I'll restart it just fine, and a few seconds later it starts again. It seems to do this untill I drive away and won't do it again untill it sits for a day or over night.

So the other day while it was doing it's surging, I pulled the air cleaner off and wiggeled all the conections of all the sensors with no change until I attempted to relocate the air cleaner, for some reason I heared a click and the engine ran smooth, until I shut off and restarted it. while doing it's surging again I attempted to find what clicked to change it's pattern. What i found was when i unpluged the vacuum line going to the MAP sensor it compensated util i repluged it... It died, I restarted it and it ran like a raped ape all day.

So reading your request and replies, I starting to feel it maybe the MAP sensor. I'm going to tinker little bit more with it while it's doing it's thing before replacing it, so i'll let you know what I find.
As well as following your jurney....