89 Chevy Silverado issues.

my oil pressure sensor sits right above the oil filter housing. its inside a cone . I dont have the one next to the distributor,. I wonder if it is bad as well. likely the original oil pressure sensor.

it would seem that unless there is a reference voltage, there would be no voltage to the tps during idle, as the Iac, coolant temp sensor, and ECM are supposed to regulate idle exclusively. I understand though that it is possible to be shorting or bleeding voltage. throttle plates should be completely closed during idle is what ive read.

Ok. I did the tps voltage test. key on engine off 4.95 reference signal from pcm, good, .0.5 volt signal from tps to black ground wire good. check black ground wire, to engine ground . good. I plugged harness back into sensor back probed and started the truck. truck was surging pretty bad and voltage at sensor was .5 volts. I snapped the throttle and the voltage increased. once rpms settled down after snapping throttle, it started idleing alot better. I then went to the tps harness connnector and started wiggling it a bit to ensure it was plugged in good, and it started surging again. im thinking that the terminals in the tps might be squirrely or the sensor pintle is mechanically sticking. im going to replace the tps sensor and see what it does.. I saw from the wiring diagram that the sensor also gets an input from the coolant temp sensor. there was no fluctuation of the voltage at idle. it remained steady at 0.5 volts.

I’m on my alldata. I’m just assuming you’re still using the 89’ computer. The map and tps share the 5v reference grey wire. The coolant sensor and tps share a ground black wire. I’m still looking for other stuff

Oil pressure switch/sensor is normally open and closes (complete circuit) at 4psi . This won’t cause a fluctuating fuel pump it’s back up if pump relay fails. You can confirm the fuel pump is getting steady voltage by a voltage drop across the 20amp fuse, in millivolts but your just looking for a drop out. I think you might be on to something at the connector from what you said. You can also unplug the iac at idle and see if it stops surging.

I removed and cleaned the TPS, and behind it where it rides in the little rounded hole track in the throttle body. I sprayed carb cleaner in the terminals and cleaned everything good. I cleaned the terminals, checked them for looseness, and put it back in. truck ran even worse. surging and wont accelerate. I rechecked the volttage from the blue wire, to the black and got .06 volts, while truck was running. specs in book say it should be .05 at idle so I dont know if that is enough difference to cause my problem. I ordered a quality TPS sensor and will replace it.
I checked the 02 sensor while the truck was running. on the 200 mv scale, it started at 138 mv and slowly dropped to around the 72 millivolts. unplugging it didnt seem to make a difference. this was unplugged, single wire sensor.
when the truck started idling really bad it was knocking and shaking around. I think it is running so lean at times it is detonating., I need to figure this out before it burns a hole in a piston.
other than replacing the tps., the only other thing I can think of is possibly a failing fuel pump, or ignition coil getting weak. when I unplugged the tps it threw a code 22. so its being monitored. intermittent problems are the hardest because I may go out tommorow and start it and it surges 3 or 4 times then idles great.

it seems that providing there is no vacuum leak, then the only things that should create a surging condition should be a bad signal by the coolant temp sensor, in my case a stuck open thermostat, a faulty TPS, or a bad ecm. The problem doesnt seem to be a miss, it ran like a scared rabbit when I took it out on the road a few days ago, temp and voltage good, and that makes me think its not a fuel pump issue either, . sometimes on stop and take off, it would stumble and die, like the idle wasnt high enough . after a good 1/2 hour of driving, and pulling into the yard, it started surging again,. not really bad, but it was there. other times when coming to a stop, it didnt surge and took off fine. these trucks must have drove techs crazy in the 90s ! ha

I was reading back through the thread and noticed where you said 02 sensor was past the cat. It should be before the cat somewhere. I can’t remember what side but I believe on the y pipe. I would use the one in your ex manifold, unless the cat is see through. On the tps anything below 1.?? Something volts the ecm assumes the throttle is closed. So if you can open the throttle a bit before you cross that threshold your gonna get that lean stumble. I had to look it up 1.25v. If you just rebuilt the Tbi gaskets and seals,orings the full kit that comes with pressure regulator but used the old injectors, they can be dribbling. And makes sure you use that little foam seal on the throttle shaft behind the tps.

Yesterday I started the truck, it was surging. it is a replicated surge, in other words it surges the pretty much exact same way, time after time, like the ECM or a sensor is attempting a correction. I put the truck in gear, drove 10 feet, then the surging stopped. Later I started it again and surging was back. Shut truck off, went out yesterday evening and unplugged tps and started it, no difference, surging was there. plugged TPS harness back in, and it seemed like it changed some, but still didnt idle right, had surge and also was missing on throttle opening. it really seems to be something going on with the TPS, even though it checks out electrically. Im hoping the new tps comes in soon to see. I just have not seen anywhere where someone has definitively said, that they replaced the TPS and it fixed a surging idle.
I dont recall replacing a foam gasket near the TPS, i will check it.
I am also wondering, if I can be getting a intermittent vacuum leak thru my Evap system. should the little sensor that sits near the front of the engine that one vacuum hose goes to the canister and one goes to a vacuum port on the throttle body be closed, or open while running. (venting or closed venting) thanks

when I bought the truck, there was no O2 sensor in the manifold but the hole was there for one. I looked under the truck and saw that there was one behind the cat. so I bought a new o2 sensor, and screwed it into the hole, because they are cheap, and I figured it would be a good place to keep a spare. Im not sure if the cat is free flowing or not, but it likely is, my exhaust seems to breathe very well.
Im going to check and see if the wire going over to the 02 sensor will reach the one on the manifold. truck is an 89, with a 92 motor. so, im not sure if the 02 sensor should be behind the cat, or in the manifold in front of the cat. As far as why there are 2, 02 sensor locations, I dont know if when the motor was changed if they kept the exhaust manifolds on it from the 92, and the 92 had it in the manifold, or they changed the exhaust and thats why. Im thinking it is more likely that when they changed the motor, they just used the whole thing, and didnt swap the manifolds.. why they didnt plug the hole is kind of puzzling, because that would send a lean mixture to the 02 sensor behind the cat, it would suck air into that hole. The truck was registered and driven until the head gasket blew abt a year before I bought it, so they must have somehow managed to make it go on down the road. ha
Unplugging the 02 doesnt seem to have any affect , at idle anyway,.in open or closed loop. not sure what function the 02 sensor has in this obd1 set up.
Im sure it does something other that throw a code, which it does if I unplug it.

Use the manifold 02sensor. If the cat is working it would look to the ecm as mostly a flat line which is not good. Unless it’s just monitoring cat efficiency, but I don’t believe that’s the case here. I could of sworn that 02 was on the drivers side manifold. Has the harness been extended or the wire to reach the back of the cat?maybe someone bought the exhaust and just made it fit where there was an 02 bung. This is a relatively simple system,so let’s keep it simple to narrow down the surging. From what I’ve read so far, use the other o2. I would probably then start it cold, does it high idle and slowly drop as it warms remaining steady? Then almost like flipping a switch start to hunt/surge? If so, it’s in closed loop when surging starts meaning the computer is doing it or trying to correct for a mechanical or input fault. If it hunts or surges when cold as well it would most likely be more of a mechanical fault. If I were standing there with you watching it surge after warm up, I would remove air cleaner and unplug idle air control and leave it unplugged and start pinching off vacuum lines including brake booster and egr. Pcv might cause a stall at this point, so if no changes,plug iac back in and shut it off,then plug pcv and start it back up. No changes? Then we narrowed it down to fuel pressure fluctuations,leaky injectors,erratic o2 ( which should be adequately compensated with the iac as it makes changes).Back in high school (when these systems were brand new) the instructor showed us how to look at the fuel spray pattern with the timing light, set it up like your checking timing and aim it at the spray pattern and you can see if it’s a clean spray or if it has any droplets that would cause surging. You don’t want any droplets. You could also remove belts as a last ditch effort just to be sure it’s not something goofy with the alternator or power steering pump(hey..weirder things have happened). You would need some test equipment to go much further. Things like checking knock counts, iac counts, o2 and such will need a scan tool. A fuel pressure gauge,timing light. You can get alldata diy for 1 vehicle 6mos or a year less than $20. I believe. This is all assuming what you said you checked is good. Curious how it turns out. To bad your not in WI.

I cannot find the place where little foam seal on the throttle shaft behind the tps. goes. there is a gasket in the kit that looks line a figure 8 with a smaller circle on top. not sure if that is it or not.
Another monkey wrench in the puzzle. my TBI rebuild kit shows some TBIs have an oring in the valve body in addition to the gasket that goes on the IAC valve. I removed my IAC and looked, and dont see a groove where an oring would seat, nor an old oring in there. I inspected the hole where the IAC sits, and recleaned everything. I also tested my purge valve for the evap cannister, it is not leaking. I replaced the vacuum hose goung from it to the throttle body, just for good karma, and started her up,, SOS same old ship.. surging idle. I can see this truck is going to make me go buy a 200.00 scan tool.

thanks im going to try everything. the reason im thinking it is not mechanical as far as base timing, valves, compression issue, timing chain worn, ect, is because there are times when it idles perfectly. engine doesnt shake, no smoke, just wonderful. so I know that it CAN run good, but ive been down this rabbit hole before chasing a fuel issue and it turned out to be ignition. I had a failing Hall effect pickup on a dodge dakota drive me nuts. it would stall with no ryme or reason, then later it would start and run no problem. after replacing fuel pump, and injectors, a guy told me to see next time it stalls, if tach moved when cranking the engine, and it didnt. oil was coming up thru the distributor seal and getting on the bottom of the hall effect pick up. (some canadians call it the HALIFAX pick up. ha

whe u say leaking brake booster, do you mean the hoses going to it, or can the booster itself leak?

also in thinking with the o2 sensor right on the manifold, it should heat up pretty fast. it might go to closed loop in a jiffy.
\there has to be a reason they put it on the manifold, its never for mechanics convienence, its function and safety. I just cant image why someone would remove the one in the manifold, leave an empty hole then hook it to the one behind the cat, which isnt supposed to be there in the first place,, haha
makes me also wonder what happened to the radiator, cooling fan, pulleys, ect all missing when I bought the truck, and the intake manifold was in the bed.
we have been going over this truck with great detail.
replaced radiator, water pump. hoses. tstat. IAC. rebuilt throttle body, new battery, new belt, new fluid in rear diff, new rear brakes and hardware, replaced sway arm bushings on front, replaced squirrly ignition switch, had heads rebuilt, deglazed cyllider walls, replaced windshield washer bottle, .. everything works on the truck, 4 wheel drive works good, clutch was supposedly replaced and grabs good. that pesky surging issue is the last major issue.

if I go start it cold, it goes DIRECTLY into a surge condition. no high idle then surge, just directly to surging. I dont have a tach, but would estimate it is fluctuating between around 200-300 on the low down, (because it almost dies) and 1200 rpms on the peak end/ over a 3 second cycle/ \if you count one thousand one , two three, it will have gone from its high to its low. so its kind of a quick cycle. just repeats , over and over like daddy playing an old hank williams record. haha

Both on the booster, it’s a rubber diaphragm inside. As long as iac and tps aren’t sucking air past is all that really matters gaskets or oring’s. If you’re 94’ still has the tbi and idles smooth I’d swap it for the hell of it just to see. I know it’s not super simple but you wouldn’t need any throttle linkage.
I would still recommend an otc genesis. You’ll get your widest range of old and new with bidirectional control. I keep a blue driver plugged in my 97’ burb but it won’t work with anything older than 96 infact I can’t think of any cheap new scaner that would support 89’ properly. I’m also not an expert on scan tools either.

if this is a clue or not, I dont kn ow but I looked in my brake fluid reservoir and it looked like a blob of algea growing in the bottom, I used a turkey baster to suck it out. iIm thinking maybe air is getting in there and letting algea grow?

Not really, in this case. Brake fluid is hygroscopic. It sucks up moisture,damages seals and rotts out steel brake components. Unless you’re having brake issues or funny things happening when you step on the pedal, just a coincidence. By pinching off or removing and capping the line at the throttle body you’re checking the line/hose and booster for a potential leak point. If the idle changes when you step on the brake with hose connected, there’s a problem.

If you have any electronic stuff, you can pop a 200ohm resister in the tps connector harness side and that would show roughly 1.0v to the computer steady. Should confirm if tps is acting funky. Keep it more than 180 less than 220ohms. Just a thought.