I really hate to bring my pickup truck problems to the f-body guys but honestly this is the most concentrated wealth of TBI know how on the net so here goes...

My 89 k1500 (5.7L TBI/auto) just picked up a very inconvenient new issue the other week, I went to start her in the morning and it fired up first turn as always (fresh powertrain has never failed me until know) and it idled smoothly as always for about twenty seconds where there was suddenly a stumble in the idle and the motor bogged and stalled flat.
Now I can start the truck most times first turn after I hear the fuel pump load up and the truck will run for 20 odd seconds normally until it suddenly dies again, I can not for the life of me understand what's going on.

Throttle position makes no difference, I can hold it wide open and it will bog down, pick up again for a few turns trying to hold RPM, and then bog again and die, sometimes repeating the bogging and struggling a couple times but then stalling in short order. When the motor bogs down it chuffs lightly back out through the TB and on the complete stall occasionally it puffs some raw fuel with force out of the TB.

I pulled a lead off and put an HEI coil tester on the end and watched my spark at idle, it jumps perfectly until the stumble where the spark completely cuts out, when it repeats the bogging and picking up I can see the spark cut out, cut in, and then cut out again respectively as the motor bogs and then picks up with throttle input.

Really not sure where to start looking sensor wise and pretty confused because like I said, it's been rock solid up until now, no problem with starts under any conditions.

Any ideas what could be causing it to start and idle perfectly fine but then die consistently in well under 30 seconds?

 

TBI is a TBI and works nearly the same regardless if it is F body or K frame.

Basic checks are in order (not in any particular order):

SES light check - do you have any codes set??? Short pins A&B of ALDL together. Read SES flashes

Fuel delivery - verify that you are getting around 13 psi of fuel pressure. You can force FP to operate with ignition. (search for instructions)

TPS functionality - verify that you do not have a flaky TPS or TPS connector. Pin A should be ground; Pin B is TPS signal input to ECM; Pin C is a 5 volt reference.

Flaky reluctor, HV ignition coil or EST module (ignition cut out) - this requires getting inside dizzy and some component testing. (search)

//RF

 

SES codes are non related, it was throwing #44 intermittently mostly at highway speeds and always at low throttle load and never under WOT (never felt a stumble or hesitation or pinging though, ever) also threw #13, I checked the O2 lead from the sensor to the harness and it was burned through to bare wire in several places and soldered at least once, I soldered a new section of wire in. I assume the #44 could well have been from the O2 lead grounding out and throwing off the reading. I havn't cleared the codes and checked if it's setting the #13 still but this couldn't cause the stalling issue to my knowledge so I didn't prioritize troubleshooting it.

Havn't checked fuel pressure but I can see the injectors clearly working away with the air cleaner off. The truck has been running like a top up until that night, WOT, cruising, idling, burnouts you name it. Not sure how something could have flaked out fuel delivery wise but I guess I should verify that I'm getting healthy PSI anyway.

I spose I can start looking at ignition as well, it's just the nature of this problem being absent on startup and then kicking in after 20 seconds of smooth sailing is greatly confusing...

 

Upon more trial and error doing a walk around while it idles and watching it stall out it really seems like I'm running pig rich somewhere during the starting/stalling cycle.

I can small lots of unburnt fuel on it's last few puffs at both the exhaust exit and around the T/B...

Is there anything that could cause regular fueling at idle and then suddenly just dump fuel in after a short time?

Edit: TPS is good, just checked it, good range from 0.75V to 4.45V

 

Check for intermittent CTS - if it opens it would cause pig rich condition.

//RF

 

I've found the coolant temperature switch pictures in my manual but I can't find the section on testing the switch itself? I'm wondering if that's enough to kill it from an idle though, like I said even if I bring the RPMs up it's like the the spark (and maybe the fuel even) just cuts out for a second or two before cutting back in but then cuts out for longer and stalls no matter the throttle position.

Edit: Ah I guess the switch is just for the gauge cluster nevermind, I'll check the ECT sensor by the t-stat.

 

Yes, CTS - coolant temperature sensor is a variable resistance device that changes its resistance vs. coolant temperature. Look for two wire connector that has one black and one yellow wire, next to the thermostat. ECM supplies a 'pulled' up +5volt reference voltage (in open circuit) on yellow wire which is dropped across CTS. If CTS looses ground connection or if it is intermittently open the ECM will sense +5 Volt at its ADC input, which is equivalent to a very low temperature (-40C). Naturally at such very low temperatures rich fuel mixtures are commanded.
At 80-85C coolant backbrobe CTS yellow wire - it should read about 1.2 to 1.5 Volts DC to ground.

//RF

 

Ok so I checked the sensor connector with ign turned on and got a proper 5V signal but when I checked resistance across the sensor pins it just read "1" at least I am almost positive I checked it correctly.

Is this a failed sensor? Possibly part of my problem?

 

To check CTS

Disconnect harness connector from CTS.
1) Set DVM to measure Vdc and with ignition ON yellow wire should read +5 Volt. You have confirmed this in your prior post.

2) Ignition OFF, set DVM to read resistance. Measure resistance between black wire and engine ground (find non painted metal surface) - DVM should be near 0 ohms.

3) Take a looks at your DVM is it auto ranging? CTS at room temperature (20C ~71F) should measure around 3555 Ohms; at 80C this value drops to 338 Ohms. At -40C CTS resistance increases to about 102000 Ohms. Most common DVM's are auto ranging, that is they automatically select proper resistance range to display. Depending on DVM when resistance is too high or there is an open circuit instrument may display error msg. I assume that you know how to use your DVM.

If you are measuring high CTS resistance and you have good test leads contact with CTS terminals then it is time for a new CTS - they are cheap about $15 to 20 in most autoparts stores.

//RF

 

Checked CTS again and resistance looked normal for the sitting temperature (58-60*), I swear I was touching the pins of the sensor properly before but I guess I didn't have good enough contact.

Any chance a clogged fuel filter or something could cause symptoms like this? I want to check fuel pressure but wont be able to until I get a t-fitting and an acceptable gauge so I'm outa luck for a few days...

 

1) Did you verify that you have good ground connection (see step #2) to the CTS??

2) You can measure fuel pressure at the fuel filter using a common 3/8" T and 3/8" SAE30R7 hose with a suitable fuel pressure gauge. This will not be as accurate as measuring at TB inlet port, but it is close enough.

//RF

 

After telling the mechanic at my work how I watched the spark cutting out the way it did he suggested the ICM might be on the way out and is failing once it sees a certain amount of heat.

This sounds promising?

 

Heavyhalf

Yes, very common - see post #2 of this thread. EST module or reluctor coil will cause ignition and injection cut outs. GM modules tend to last longer, but aftermarket are all over the place. We had another thread a while back were symptoms were nearly identical - too much silicon heat compound under EST module resulted in premature failure due to over heating (this was in combination with aftermarket coil which was drawing a lot more current).

//RF

 

Well I may be wrong, and I usually am, but my bets on the module I removed to check today, corroded contacts aside it was only held down with one of two screw and it's easy to tell which side of it lifted and was getting pretty hot once the grease all evaporated... hopefully this is as serious as my problem gets.



Knock on wood...

 

Better auto parts stores have ability to test EST modules. Before you re-install old or new module make sure that the mounting surface is clean from old heatsink compound remains, there is no dirt and that the mounting surface is flat. Basically, you have to give a new module every opportunity to transfer heat from its semiconductor devices to dizzy housing where it can be dissipated.

//RF

 

I'm having the same problem with my Firebird, thought it was the CTS but was fine, did you find out if it was the module?

 

Honestly it's so easy to pull your dizy cap to check if it is so I'd recommend checking it visually anyways, and like RFmaster said some parts stores and some shops will have the tool to test and see if it works properly if you've got the connections or ask nicely enough.

Mine was obvious where the heat was frying it out. And with a new module she runs fine..

 

I checked it yesterday, but there weren't any visual indicators. Since I'm having the exact problems you are, I was just wondering if replacing it helped. Been going back and forth between the store, buying and returning parts that I thought were the problem. I'm going to try the module today. Spent all day yesterday trying to find any vacuum leaks, tested all the sensors (all were good). No codes were showing.

Hopefully it's my module as well.

 

It CAN be other things like fuel pressure, but if it idles smoothly and revs up fine but only for 30 seconds my bets on the module. Other parts of the dizy can go bad too hopefully it's the easiest one to replace in your case. Do you have an HEI coil tester so you can watch your spark? Pretty cheap tool and very useful for testing ignition related stuff. I suggest getting one (less than $10) but it HAS to be the HEI one. That way you can start the car and watch the spark and see if it cut out like mine during the spots where it starts bogging.

Keep me posted, hope it works out.

 

I replaced the module and it ran a little better, but still dies. Was in the process of checking the plugs (covered in gas, smelled funny) and broke one. Going to get wires, plugs, and dist cap/rotor. It needs a tune up. I don't trust those wires, even if they are ok, resistance-wise, they were tied together and some of them have the insulation starting to "rub" away.

Also testing the fuel, making sure the PO didn't put diesel in it, or bad gas that has sat in there for awhile. It's not igniting, so one of the two are at fault.