5.7 Tbi cutting out...

[Laderhosen]

I have an ’87 K5 Blazer.

Whilst driving the other day, it cut out on me, and wouldn’t start at all. I did the following:

1. Checked the fuel pump was working, was priming ok, but as soon as the ignition was turned on, the engine wouldn’t start. As soon as it was on, the pump would stop pumping.
2. I bridged the fuel pump relay with a wire and fuse, and the engine started, but when revved it misfired.
3. Replaced the fuel pump relay - didn’t solve the issue, started, began to drive, but as soon as I lifted off the gas stalled.


At this stage I’d ran out of skill, so I took it to a mechanic. They plugged the ECU in, which threw up a fault code about the temperature sensor (on top of the engine block at the front by the fan). They also changed the fuel filter which they said was completely blocked. These were both replaced, and they test drove it and said it was fine.

I then collected it, again it started up fine, idled fine, drove about 1/4 mile up the road, then as soon as I lifted off the gas it cut out. Starting it up was a struggle too, it didn’t wanted to start up straight away, but after a few attempts it fired up, and I drove again for a very short distance and it cut out. So I took it straight back.

They called me yesterday and said that they’ve ordered a new ignition module.

Is this likely to be a problem? 

The guy I bought it off said it had an intermittent fault which he reckoned was an ECU fault, though I’ve had it a year now and its only cut out on me once, when I had left it idling for about 10 minutes. 

[topp]

The ICM tends to do what you are describing- It controls Fuel pulse through the TBI.
It is normal when you turn the key for the fuel pump to work for 3 seconds, then cut off. If the engine starts it will turn back on....

[zieg85]

Sounds to me a weak fuel pump with a clogged sock filter... My $.02

[big bear]

i agree with zieg . definetly sounds like that

[Laderhosen]

Sounds to me a weak fuel pump with a clogged sock filter... My $.02

Is the sock filter on the fuel pump itself then???

I don't think it's the ignition module either.

When I picked it up, with the new fuel filter, if the ignition was turned to position 3, for maybe half a second, the first two times it wouldn't start, then the third time it'd burst into life. Symptom of a weak fuel pump???

[Laderhosen]

Topp, have you had a similar problem?? Did it turn out to be the ICM???

[zieg85]

Sounds to me a weak fuel pump with a clogged sock filter... My $.02

Is the sock filter on the fuel pump itself then???

I don't think it's the ignition module either.

When I picked it up, with the new fuel filter, if the ignition was turned to position 3, for maybe half a second, the first two times it wouldn't start, then the third time it'd burst into life. Symptom of a weak fuel pump???

The sock filter is on the intake of the fuel pump in the tank


Typically yes a weak fuel pump.  Another thing is if the fuel filter was as clogged as they say it was it could have weakened the fuel pump even more.  Your symptoms sounds just like a van I worked on for my brother in law.  The fuel pump all did the right things as far as operate the 3 seconds and shut off but it was to weak to keep it going down the road.  The true test would have been if it was the ignition module you would have lost the spark.  If it was the fuel pump you could have squirted some gas down the TBI and it would have started back almost immediately.  

[zieg85]

Another thought... I know in the case of another vehicle I recently had a 92 GMC with the TBI, I had a bad temperature sensor.  It would start up but idle real low, and after a while it would race the engine all by itself when the engine was at operating temperature.  Put the new sensor in and the check engine light went off and the truck started great all the time.

[Laderhosen]

It has spark, and when it initially stalled and wouldnt restart (before the fuel filter was replaced), I sprayed some 'Easy Start' into the tbi and it fired up fine, then stalled after a short while - indicating, in my opinion, the fuel pumps the problem???

Sounds to me a weak fuel pump with a clogged sock filter... My $.02

Is the sock filter on the fuel pump itself then???

I don't think it's the ignition module either.

When I picked it up, with the new fuel filter, if the ignition was turned to position 3, for maybe half a second, the first two times it wouldn't start, then the third time it'd burst into life. Symptom of a weak fuel pump???

The sock filter is on the intake of the fuel pump in the tank


Typically yes a weak fuel pump.  Another thing is if the fuel filter was as clogged as they say it was it could have weakened the fuel pump even more.  Your symptoms sounds just like a van I worked on for my brother in law.  The fuel pump all did the right things as far as operate the 3 seconds and shut off but it was to weak to keep it going down the road.  The true test would have been if it was the ignition module you would have lost the spark.  If it was the fuel pump you could have squirted some gas down the TBI and it would have started back almost immediately.  

[zieg85]

Sounds like the fuel pump to me as well then.

[Laderhosen]

.... However saying all that, when the engine is cranked but fails to start, there's a smell of unburnt fuel, which would suggest the pump is pumping?

Also, it seems as though it only cuts out when coasting, and not under acceleration?

I spoke to the garage yesterday, he said ge took it out for a 20 min test drive and it didn't cut out at all. I'm now wondering if it is the ICM, as it was fairly cool on Friday, and on the days it's cut out straight away it's been round 85 degrees. Have heard the ICM can fail when it's hot?

[Lt.Del]

next time it cuts off, you must check for spark.  That will tell you an awful lot. Fuel vs. ignition.