i've got a 91rs with a stock 350 vortec with the gmpp intake to use the stock tbi.it's only mods are an accell billitech dist. and hedman shorty headers and dynomax catback.when it's cold it takes a little longer to start than a tbi car should and it stalls unless you keep it running.it also hesitates real bad when it's cold.as soon as the car gets to about 150-160 degrees it stays running fine and the hesitation is gone.it doesn't set any codes.the fuel pump died once and i replaced it with an ac delco pump and new fuel filter and it started back up but still had the stalling and hesitation problem.i've finally got the car almost road worthy now.i just got the vats bypassed,running on all 8 cylinders(bad #7 intake valve)and the cooling fan working correctly now.i'm so close i can feel it so any suggestions will be appreciated.i do have a spare ecm that i can try and a spare tbi laying around that i haven't tried yet,thanks

 

You need to check the fuel pressure, you might even need to get an adjustable fuel pressure regulator. The vortec heads move alot more air/fuel then the old set up, did you get 350 injectors?

 

so the valve is still bad or you fixed it?

could possibly be a coolant temperature sensor problem.

my car runs rough until the o2 sensor warms up, but im not stock and im tuning it.

 

i fixed the intake valve.it just runs like crap until it's warm the it runs like a champ.i even put a new ac delco o2 sensor in it since it had the original 209k mile factory one but it didn't help.

 

try unplugging the coolant temp. sensor. it is located on the intake manifold front, to the left of the thermostat housing, if looking at the car standing in front. try when cold to see if it changes anything.

 

If you have a DVM (Digital Volt Meter), you can check the sensor. This wont rule out a bad lead or connector, but it'll give you a rough idea if the cool temp sensor is still good.

 

i've got an older snap on brick style scanner.i also thought coolant temp but it appears to be showing the right temp when it's cold(usually 60-70 degrees when i start it).i'm going to try the spare ecm and tb this weekend and i'll report back.

 

i couldn't find my spare tb to try.i did check the tb spray pattern and the driver side injector has a lot of spitting coming out of it.the spray pattern on the passenger side looks much better.does anyone know of a good source for injectors?i'll probably just go ahead and get 2 new ones for a 350 and then get a chip for it.the original injectors have 210K miles on them and nothing lasts forever.

 

I had the exact same problem!!!! your coil is bad!!! Cold starts took for ever and it would hesitate only when cold, Tested coil and it was bad!!! Once i replaced it my car ran like a champ!!

 

It's funny how people start throwing new parts at cars when it is giving them problems, people it's so much cheaper to take to technician!!! After purchasing so many parts it comes out the same as taking it to the shop for a diagnosis!!! I know this because i am studying automotive and people always bring me cars that have many unneccesary new parts that are unrelated to the actual problem. Good luck bro and your symtoms are ignition related and if it was the coolant temperature sensor your ECM would have thrown a code. Does your car have a miss while it is warming up and does it hunt for idle too???

 

A cold coil? Wouldn't a coil get worse as the engine bay got hotter? My TPI is doing the same thing. Even when it starts to run good, then shut the engine off & restart...It idles rough for the first 30 seconds or so. All that leading me towards a Closed/Open Loop diagnoses of a sensor.

 

Sounds dumb, my friend when you are diagnosing a problem like that you need to test all ignition components before you start spending rediculous amounts of money buying unnecessary parts. Do you have access to all data? All data is a very useful tool when diagnosing intermittent problems, and it also has diagnostic charts which will help you pin point the problem. Anybody can turn a wrench but a true technician solves complicated problems that seem to puzzle the novice pepper tree mechanics out there. Yes a coil will act up when it is cold, and i would also test the ignition module too, Are the injectors spraying fuel??? If so than it must be a ignition problem, A bad ecm is a possibility. You must go through the diagnosis chart to pin point your problem!!!! Go to autozone and purchase a years subscription of all data for 29.99

 

Don't talk down on us. You do not know us, nor what we are doing/have done to diagnose. We give a grief description, not a complete 100% step by step of every minute detail we have done, just what we know is relevant to the issue.


I have not been randomly replacing parts "thinking" they were the cause. Just a couple of small parts that had broken connectors possibly causing bad connections & bad readings. 2 parts total. 1 broken sensor & 1 broken sensor pigtail. Whether or not the parts were truly bad, they needed replacing because they might have been affecting the connection.

My complete distributor is new (ish). It had to be replaced last year for a different issue, nothing related to this. I have noticed that since replacing the broken CTS, the MAF code no longer comes up & node for indicating a bad CTS (if there is such a code) never came up at all.

 

You need to get this thread moved to the tpi section, not the tbi section. You might get more help there.
On a cold engine you need alot more spark then a hot one, b/c the a/f ratio is alot richer.

 

The OP has a Vortc long block under his stock TBI, so he posted in the correct section.

I have a TPI, but a very similar situation & have posted in the TPI section but minimal responses. I only asked about the coil issue that one member posted about, which would be the same remote coil between TBI or TPI.

 

Testing the Ign coil is fairly simple. Test it for max KV using a cheap adjustable KV tester from any parts store. You can also test it for primary and secondary resistance using a DVOM. The primary side should read .5-1 ohm. The secondary side should about 27Kohms. However, the KV test is the best and quickest way to determine coil health. There are a few things that I would look at for causing this problem aside from a bad coil. First would be fuel pressure and the fact that you are running a modified system. Use your scanner to read data as the engine warms. Does the O2 sensor tend to the lean side as it warms up? Once the system reaches closed loop, what does your BLM read? Is it above 128? That would indicate insufficient fueling.

Remember, on engine startup, there is no closed loop fuel correction. The system relies on a programmed cold start fueling map. Any changes to the engine from what the ECM was originally programmed for will cause issues on cold start. Once the system goes into closed loop, the Long and Short term fuel trims compensate atleast for minor changes.

Edit: A question for Undercover TBI: Does the ECM have a default map for the ECT sensor? This has not been my experience. I have found that when the ECT is unplugged the engine tries to run like it's -34 degrees outside.

 

Yeah I know that. -34 or -40 is what the snap on scanner shows when you unplug it. I just said to try that and see if it starts, not saying it'd fix its problem. if the sensor is reading wrong and telling the engine it is 220 degrees or whatever temp, putting a different temp to it may make it start. he may also be able to find a bad connector which is common. I have seen it before. A car starts with the sensor unplugged but not plugged in. So it is a possibility. (starts, but does not run good)

 

all i was asking is did anyone know of a good source for injectors.i don't have anyway of checking fuel pressure on a tbi because it doesn't have a schraeder valve and my fuel pressure kit doesn't have any adapters for tbi.one thing i did notice is it starts and runs much better with the stock breather off the tb.i even tried taking the filter out but it didn't matter unless the whole breather was off.

 

Southbay, injectorsplus, dctrumpet has a ebay tbi parts company. They sell an adapter that goes on your fuel line that you can check the pressure. check out the "vendor feedback secton" on the site, type in injectors and see who pops up. thier is one more I cant think of.

 

half the problem is fixed.the long crank and stall was because of a bad battery.it turned over fine but it would eventually start and stall several times.i replaced the battery and it starts fine and no more stalling but i still have the hesitation.i replaced the 2 injectors with gb remans and the spray pattern looks good.i think it has a vacuum leak or a bad map sensor as after it warms up it surges and idles alittle high sometimes.i haven't been able to find the leak yet though.it's slowly getting closer to the roadworthy status.

 

Vacuum leak is likely culprit but be sure to verify solid spark with no drop out on acceleration and timing set correctly. Also, TPS can cause hesitation if it has a glitch.

 

i just got my ac delco tps and map sensor today.i'm going to put them on tomorrow.the timing is correct at 0(i've tried it at 2,4, and 6 degrees with the same results).i even blocked off the egr valve to no change.i thought vacuum leak but so far haven't been able to find one.hopefully the tps or map will fix it.

 

i replaced the map and tps with ac delco pieces and same problem.i also tried my spare ecm,same problem.when i tried my used ecm it actually throwed a code for exhaust lean 44.i've checked everywhere for vacuum leaks but none found.i ordered a fuel gauge adapter and gauge this morning to check fuel pressure.when i replaced the pump i didn't replace the hose in the tank and i'm thinking that may be the problem.

 

If you find low fuel pressure, be sure to dead head test the fuel pump before you drop the tank again. While I have seen the rubber section or pulsator in the tank fail, I have more often seen the regulator spring in the TB fail.

 

i replaced the section of hose in the tank(it had swelled up)but i still have the same problem.i got a fuel pressure gauge hooked up and it shows 9-10 psi but it still drops pretty fast when you cut the key off and it seems to take a little longer than expected to build enough pressure.the pump is an acdelco unit.i'm with you on that the fuel regulator spring may not be right(it's original 210k mile)so i may just get an adjustable regulator and try that.the car runs great wide open just hesitates when you first pull out or when your cruising at 50-70 mph.

 

I just had a '90 1 ton van in the shop with the 7.4 engine(same FI and ignition system as your RS). Had a terrible rough idle and was hardly drivable when first started cold. I was sure that I would find a vacuum leak or other air/fuel related problem. I checked the fuel system thoroughly and found no real trouble, other than that the pressure does drop off quickly after key off. Fuel pressure was 11.5 psi. Tested for manifold leaks with propane.

Checked the distributor cap and found that the arc spots on the cap contacts were uneven. Normally, the spot on each contact will be at the same point on the contact. Generally on the leading edge. On this cap, the spot on half the contacts was at a different spot than on the others. The only thing that could cause this is a fault in the timing core and pole piece, causing the PIP, and thus the spark, to occur at a different point on some cylinders. The pole piece and timing core were badly rusted. The ign. module is a fairly new GM part.

I replaced the distributor with a known good test part, transferring the new GM module to the replacement distributor, and the engine started and ran great the next morning when fully cold. Problem solved.

 

problem solved.i replaced the regulator with an adjustable one and set the psi to 13 and it runs with no hesitation.even my injector pattern looks better.now maybe one day i'll put 350 injectors and a chip in it.thanks for all the replys.

 

ASE

That's a great tip - analysis of a bad ignition system - dizzy. I have of few of these rusty old distributors in my collection courtesy of swap meet, but now your explanation makes perfect sense.

//RF

 

How many times have we chased a problem that we swore was fuel related, only to end up finding a bad distributor. I want to remember this in hopes of saving myself and others this frustration.