tbi idle and starting issues

[07coupegt]

Ever since buying my 91, it has had to crank longer than normal to start after sitting for a while (I am assuming that it isn't priming properly). Lately though it has developed idle problems. The truck usually idles good when cold, and always runs great when stepping on the gas, but once it warms up, the idle is slow and tends to surge up and down. So far, I have replaced the plugs, wires, cap, rotor, pcv valve, and Iac. The Iac greatly reduced the problem, but it still does it from time to time. Then a couple days ago, when I started it in the morning, it was idling fine, but then suddenly revved up to 1800rpm, and didn't want to go down, below about 1500 until it warmed up. Tonight after work, it was -33 degrees (the truck was plugged in). Tt started okay (had to turn over for quite a while before it fired), and was idling at about 1200, like it usually does. About two minutes later, while I was scraping my windows, the truck stalled without warning, but restarted easily. Any idea what is causing this. This is the first time the truck has ever stalled, and I don't want to end up stranded.
fuel pump relay maybe? or coolant temp sensor?

 

[lectric80]

If you are having to crank it that long, I would be looking at the fuel pump itself. Can you hear the pump start when you turn the key to the on position without cranking? If not, then you need to look at the relay, otherwise I would suspect that the pump is wearing out and not building pressure fast enough to start the truck normally. TBI systems don't require a huge amount of pressure, but until it gets enough pressure to the injectors it isn't going to start.

 

[07coupegt]

relay

Before starting the truck this morning, I rolled down the window and turned the key to the on position, and it sounded like the fuel pump was running. I guess this means that the relay isn't the problem. Would a plugged fuel filter cause the engine to crank for longer than normal after sitting for a while? At one time I had an astro van with a 4.3tbi, that would take nearly a minute of cranking to start when cold. Mechanic said it was caused by the check valve in the fuel pump leaking, causing it to loose fuel pressure when sitting. Could my blazer have the same problem?
As for the truck stalling last night, I am not sure what caused it. The truck idled fine this morning (it is still -32 here ). Is it possible that a fuel line had started to freeze, causing the motor to suddenly shot off before?
Thanks.

 

[Thunder]

First thing you should do is replace the fuel filter. Cheap, easy, and should be done once a year. See how it runs after that. post up- if still having problems.
Have you checked for trouble codes?

 

[07coupegt]

As soon as the weather warms up a little, I'll change the fuel filter. I haven't checked for codes, but the SES light never comes on, so I don't think there are any. I wish I had a scanner, so I could see what the tps, map, and ct sensors are doing.

 

[Jessie James]

The shop manual for a 1990 says check these:

Vacuum hose for splits kinks and proper connections

Air leaks at throttle body mounting and intake manifold

Wiring for proper connections (important)

Motor mounts for damage or grounding out on frame

Throttle linkage sticking or binding

Ignition timing

Alternator output 9v or better

P/N (park / neutral?) switch circuit

PCV valve - place finger over inlet hole - valve should snap back. If not, replace

A/C signal not turning on

Controlled idle speed (chapter 4...)

MAP sensor. Start and idle engine. Disconnect MAP electrical connector. If idle improves, replace sensor

Injectors leaking - check fuel pressure.

If a sticking throttle shaft or binding linkage, causes a high TPS voltage, EMC will not control idle.

Vacuum leaks.

Battery cables clean and secure.

EGR

Contaminated Oxygen sensor.

Hope that helps!

 

[lectric80]

Unlikely to get fuel freezing at -32, diesel yes but gasoline usually won't start freezing at that temp.

Yes the check valve could be allowing fuel to drain back, requiring a longer run time of the pump to get to pressure.

I had a 88 Astro that would die occassionally when you would come off the gas. It was the fuel pump not able to maintain the pressure, or something, but replacing the pump solved the issue.

As others have said, if the filter is plugged enough it could restrict flow and cause a hard starting issue.

 

[tRustyK5]

if you're not getting a check engine light you can scratch the coolant temp switch off your list. I agree on swapping a new fuel filter and seeing how things are...

Rene

 

[07coupegt]

SeS light

Funny you should mention check engine light. The truck has been running good (although still taking a little longer to start), but today while driving it, the SES light came on. When I got home, I shut the truck off, and when I restarted it the light was gone. Guess I'll have to figure out how to check the codes on it.

 

[79rustyk10]

I had a pump go bad, from the outside it sounded good. When I pulled the sending unit out and turned the key on you could hear it was hella weak. New pump and the issue went away.

 

[07coupegt]

I checked, and got code 42, electronic spark timing. My service manual says to check for bad connection at ignition module or replace hei module. Unfortunatly, I can't get the hood to open now (frozen latch?).

 

[smier]

Check out Thirdgen.org and go to their TBI section, lots of info and you can find how to check your codes without a scanner. This is the OBD1 computer setup so they are sometimes hard to diagnose correctly, due to the limitations of the computer itself. For example, I have an 86 camaro which was hard to start, idle would jump around and sometimes stall... Scanned for codes and it spit out one for th MAF sensor, check the MAF and it's good. Cleared the code and ran great for a day. Next day same thing, now it spits a code for Throttle position sensor, replace it and the IAC. Cleared the code again, ran great for a couple hours, then back to a MAF code and runs awful... 2 months of checking and replacing parts to find out the ECM itself is bad. I was relying on the computer to diagnose itself, it took it a while to figure out it was the problem. These old computers are known for spitting a code for one sensor, when the real problem is another sensor malfunctioning, throwing the readings out of whack. Don't just throw parts at it, you'll find it gets expensive real quick. Take your time, check the simple things first, a bad ground has been known to wreak havoc on these fuel injection setups.

 

[smier]

Just saw your post about the code, just wanted to add the best way to check your fuel pump is to actually check the pressure with a guage, I'm not sure on the tbi setups, butthe tpi pressure is usually around 40 to 44 psi.

 

[07coupegt]

I changed the fuel filter last night and the truck seems to start slightly easier, but tonight (-33 again) I started the truck and it was running fine, but then after about a minute it suddenly stalled and was really hard to restart. When it died, the ses and the battery light came on, although the lights didn't dim at all (and the voltage seems normal). When I did get it restarted, the truck was missing for the first couple seconds, kind of like it was flooded. Any ideas? I checked the connections on the hei module last night as it suggested in my service manual. Bad ignition modue? Think the code i got is part of the problem?

 

[smier]

To check the ignition module to see if the solid-state switch is capable of switching when the 5 volts are applied, the engine should not be running and the ECM connectors should be disconnected. An ohm meter and test light can be used. With the ohm meter on the EST circuit, it should read less that 500 ohms (in some cases, a lot less). Using a test light to battery voltage, probe the bypass wire. With this voltage applied to the bypass wire, the solid-state switch inside the ignition module should switch and the ohm meter on the EST circuit should go over 5,000 ohms. There is also another way to check this with the engine running: by removing the bypass and the EST wire from the module or ECM, run a jumper from the reference wire to the EST circuit. The reference signal is the signal that is used in the bypass mode to trigger the primary coil voltage. Apply 5 volts from one of the 5-volt reference circuits or a test light to battery voltage. This voltage on the bypass wire will activate the solid-state switch, in turn switching the module. On some models, the 5 volts or test light will have to be applied before starting or the motor will stall when the voltage is applied to the bypass. If the module is switching OK, the car will continue to run on the reference signal. If it stalls, the ignition module is not switching properly.
I copied the section above from an article, do a google search for "gm code 42" or " that annoying code 42" and yo will find others, hope this helps, but it sounds like it could be the ignition control module possibly.

 

[07coupegt]

I replaced my leaky rad tonight and installed a new ignition module and the truck now runs better than it ever has in the 4 months I've owned it. It now idles perfectly and has noticably more power. The truck seems to be starting good too, but it was parked in a heated shop, so I'll have to wait to see how it starts when cold. I also no longer have the code 42, even after driving for an hour, and starting the truck several times.
Now all thats left on the to do list for this week is my lift kit and axle swaps.