Have 1990 1500 5-speed manual. Swapped tired 305 for rebuilt 350 a month ago. Moved everything from old motor to new one, manifolds, TBI, dist etc. Set timing to 0 degrees. Put in new 350 knock sensor, 350 injectors and a Hypertech chip. (Also new plugs, dist cap and rotor)The engine runs great when its cold. Starts easy, smooth idle and good aceleration. The problem is after it warms up. The idle becomes rough (and low, sometimes have to tap acelerator to keep it running) and it will occasional die at a stop. (starts right back up) Has a near constant hesitation during aceleration and at highway speeds and is burning a lot of gas. I verified the flap in air cleaner is opening and changed ESC module. EGR diaphram is free. I get no codes. Funny thing is, the old 305 had the same sort of problem, so I know something I moved to the new motor is causing it. It seems like a timing issue of some sort since it runs well cold, but I'm at a loss... Anyone have any ideas????

O2 sensor or TPS could be suspect. Did you use the ECM from the 350 ?? The IAC can also cause the symtoms you are having.

I did change the O2 sensor, forgot to put it in the post. The ECM for the 305 and 350 are the same. Tried one from a buddy's 350 just to rule it out as being bad anyway though...

Well, if it wouldn't for you saying it did the same thing with the 305 I'd tell ya to buy a 160 degree thermostat (if you already haven't) becuase you are supposed to run one of those anytime you use a chip.

Try the throttle position sensor (it screws in the throttle body). It might be causing your problems.

This is something simple, but if you buy your gas at the same station, try a new one becuase you may just have bad gas.

Changed TPS and MAP. Slightly better idle when warm, but still low. No more dieing at stop signs or traffic lights. Decided that maybe the timing must not be advancing correctly, so I pulled the distributor this weekend to change the pickup coil and ignition module. While disassembling I found the shaft would not pull all the way out of housing and the reluctor (the magnet) was cracked across the bottom and would turn on the shaft without too much effort. Got used dist from boneyard and put in the new parts. Reset timing. Has better power up thru the gears and actually accelerates much better at highway speed and up hills (before at 55mph, you could floor it and it would basically do nothing and you could not go up the slightest incline without down shifting). It STILL stumbles during acceleration even though its picking up speed and RPM much better. It's almost like a miss at highway speed. I think it's doing it up thru the gears too, but its not as noticable as it is in 5th gear. Truck will actually shake like a tire is slightly out of balance, but if you back off the throttle a little while its stumbling and shaking it will smoth right out. Pulled plugs and regapped (all were new a month ago and looked good, no fouling) and replaced ignition wires. No different. Replaced ESC again with the used one I tried before and it actually made it a little worse. P/N is different on the used one, so I'm not sure if that was a good indicator or not. Trashed it anyways. As previously posted, I had changed the ECM with a buddys, but that was before I changed the chip and injectors. Maybe I'll have to try that again... For sh*ts and grins, I hooked the timing light back up without disconnecting the wire just to see what it does. At idle it's running about 8-10 degrees advanced and when you gas it to about half throttle it advances further, but if you then push it to WOT (or from idle to WOT) it retards to near 0 degrees or below before advancing back up some. Does anyone know if this is normal? There isn't any engine knock that would cause the ECM to retard the timing. Anyone have any ideas.

Could the wires going to the knock sensor be pinched somewhere or cracked or burned???? Also, did you go through the throttle body at all, maybe a rebuild could help or some new injectors? Other than that, nothings coming to mind. :dunno:

shouldn't the timing be at or around 8 degrees advanced at idle, that is how my 88 is and it runs great. And running bad at warm sounds like a bad coolant temp sensor. You might have 2 of them on your engine.

I was wondering if you had fixed your problem yet rpreston? I'm having the same exact problem and we had it on the scanner and it wasn't throwing any codes, have yet to even see a "service engine soon" light yet. My brother is a very good mechanic and knows these trucks like the back of his hand. He says the only thing that he thinks is wrong with it is that the computer is going bad. Mine was losing it's get up and go after awhile and it was just rebuilt a little over 20k ago. So, I brought it over to his house and we put a set of vortec heads from a '98 motor, vortec aluminum intake, 100-150 nitrous kit, 21/2" custom true-dual exhaust w/ Magnaflow 21/2 dual in/out muffler, MSD electronics and a few other little things. As soon as the truck goes into "closed loop" it runs like crap, exactly how you describe it.

I will be putting in a new computer here in the next day or so and hopefully that's what solves the problem? I did what he told me and pulled it out, slapped it around a little and put it back in, boy was that an eye opener! It ran awesome, totally different idle and the way the thing accelerated was unreal! But, she's back to being stubborn again, so the bad computer thought may be the culprit after all? I'll let you guys know when I get it changed out.

Good luck with yours!
Bobby

Hope the new ECM takes care of your problem, Bobby. Last weekend I borrowed my buddy's 90 truck again and swapped in his ECM and the ESC module and neither made any difference. Before that I checked the wiring from the knock sensor to the ESC and from the ESC to the ECM. I also monitored the voltage signal from the ESC to the ECM. There was no change in voltage at any time so the knock sensor circuit is working fine. I even monitored the the output signal from the ECM to the distributor. It was a nice even signal increase as the timing advanced on exceleration. I also hooked up the timing light to his without pulling the wire and his timing did the same exact thing as mine, so that must be normal. I'll try the coolant sensor and maybe even IAC valve for the low idle... I'm running out of things to try. Could the CAT be clogged and causing this? The old 305 was burning a lot of oil...

So far I've changed ECM, chip, ESC, Knock, O2, TPS, MAP, Injectors, rebuilt the distributor, New plugs and wires. Also ohmed out coil. No codes or SES light...

Well CRAP! I was hoping that the new computer would have fixed your problem! Is your's throwing any codes or shining the "service engine soon" light? Oops, just noticed in your post.

Mine has nothing, runs great in open loop, but as soon as it goes to the sensors......embarrassing to say the least! Well, I'll let you know what happens when I change mine out, didn't get it in the mail today so maybe tomorrow?

Good Luck!

Have you taken it to have a dealership look at it? What a stumper! Now, was your buddy's truck the same as your's, as far as engine size, tranny (manual or auto) same computer model #, etc?

This is really bothering me and I'm sure you too! It's weird that once it goes to closed loop (reading off the sensors) it goes to crap. You would think that the computer would be the culprit! If this doesn't work, I'll have my brother check with the dealership and see if they have any knowledge of this kind of action.

Sorry, this just bothers me, was hoping that you would have been running fine now rpreston!

BTW, before I did the latest work, my brother thought that something was holding it back. We whacked off the cat thinking that it was bad, had a lot of soot, carbon chunks when we cut it out. I had it rebuilt about 20k ago and it doesn't burn or leak any oil, the motor and oil is clean as a whistle.

I'm sure that you did, but did you check/change your fuel filter?

well, I pulled off the egr valve and partially blocked off the egr port (I say partially, because the stainless I bought was a little too thin and couldn't get a good seal). It's running a lot better now. I have an egr block-off plate coming from Summit, so should be a lot better when fully blocked? It idles a lot better, doesn't stutter as much under acceleration, no surging, etc.

I'll let you know if this works. Thanks, Bobby

Hey Bobby! Thanks for the posts and concerns. My buddies truck is also a 90 with a 350, but has an automatic. The ECM is the same as mine, but I was putting my chip in it. I haven't taken it to a dealer or mechanic. After all these years of doing my own maintenance and engine swapping it's probably becoming an obsession to figure this thing out, you know, can't let it beat me sort of thing...Ha

Anyway, I'm going to try the coolant temp sensor and IAC this weekend. I'll let you know what happens.

Glad your's is getting better!

Rick

BTW. I did change the fuel filter awhile back, before the engine change.

Well cool Rick, glad to see that you are still trying to beat it and not let it beat you. You know I've seen a few people over the internet that are having the same problems, but nobody seems to come up with the answer yet or doesn't post it lol.

Hopefully, I get the egr blockoff plate tomorrow in the mail, I'm probably looking at monday for the new computer :( I'm not even sure if it's the right one, my mom had it packaged up so she couldn't read me off any numbers. It's running a lot better with the egr blocked off, but there is still air pumping out from the side, so I'm hoping that once I get the plate it will block off all of it and make it run that much better. We'll see, I guess?

I just noticed that you had a "hypertech" chip too, there is another forum I visit called "fullsizechevy.com" and the hypertech is like a 4 letter word there lol. Well, good luck with the new parts, try one at a time and see how it goes, maybe knock it out and get your money back for the 2nd part? My brother wishes that he could have had it a little longer so that he could have hooked up his scanner and watched it as it went to crap to see if it would pinpoint something. He said that while it was on the scanner, nothing came up as unusual, everything was in working order, not sure if it had entered "closed loop" yet though.

Oh well, good luck buddy and I hope you beat it this weekend!

Bobby

not to keen on newer models but could it be all that emissions crap??? may b try takin all that off?? acidently owned a ford once andhad simaler probs

Hope the new ECM takes care of your problem, Bobby. Last weekend I borrowed my buddy's 90 truck again and swapped in his ECM and the ESC module and neither made any difference. Before that I checked the wiring from the knock sensor to the ESC and from the ESC to the ECM. I also monitored the voltage signal from the ESC to the ECM. There was no change in voltage at any time so the knock sensor circuit is working fine. I even monitored the the output signal from the ECM to the distributor. It was a nice even signal increase as the timing advanced on exceleration. I also hooked up the timing light to his without pulling the wire and his timing did the same exact thing as mine, so that must be normal. I'll try the coolant sensor and maybe even IAC valve for the low idle... I'm running out of things to try. Could the CAT be clogged and causing this? The old 305 was burning a lot of oil...

So far I've changed ECM, chip, ESC, Knock, O2, TPS, MAP, Injectors, rebuilt the distributor, New plugs and wires. Also ohmed out coil. No codes or SES light...

Sorry Rick, just noticed the statement about your cat. You may try rapping on it to see what kind of sound it makes. It may be clogged and probably is if it's the original. I cut mine out since we don't have to be smogged in Washington and it was full of crap! I was blowing chunks of carbon out of the pipes for a while afterwards, probably screwed something up having it in there? If you have to have one where you're at, you might try gutting it, not sure if that would work anymore? Or buying a new one, high flow maybe? Just thought I would throw that in there, just in case that was your problem?

Again, good luck!
Bobby

Thanks to all who read or responded to this post. The final piece of the puzzle was a bad coolant temperture sensor! Running good now. Again thanks!

Rick

hello fellas,having the same problems.Where is the coolant temp. sensor located on the 350 ci.Mine is also a 90 model.

Thanks to all who read or responded to this post. The final piece of the puzzle was a bad coolant temperture sensor! Running good now. Again thanks!

Rick

hmmm, they're cheap and might be something to try myself? Mine is running a lot better since pulling the egr and installing the block-off plate, but still doesn't seem to be up to par. I think I will go buy one tomorrow after work. Glad you got her running good Rick!

Bobby

hello fellas,having the same problems.Where is the coolant temp. sensor located on the 350 ci.Mine is also a 90 model.

I don't have a stock '90 intake, but your's should be off of the intake manifold, pretty close to your thermostat housing, maybe right next to it. It will probably be brass, threaded into the intake with a couple of wires coming out of the end.

It looks like this: http://www.partsamerica.com/PartDetails.asp?SourceArea=SHOP&SourcePage=SEARCHRESULTS&MfrCode=EVP&MfrPartNumber=4963&PartType=290&PTSet=A

It has a waterproof connection that snaps onto the black plastic part of it.

Hope this helps,
Bobby