1988 GMC rough idle, no power, stalls

Hey guys,

I recently got a 96 gmc and im working through some problems to get it on the road. Im also in Australia so replacement parts are hard to get and slow to arrive so i need to make the best assessments i can.

Anyway truck is 5.7L manual, stock.

It is hard to start, requiring a lot of turning over to do so, it runs for about 10 seconds, you can almost time it, and then it stalls, meanwhile idle is rough, seems to hunt/miss a little. If i raise revs, keep it running and drive it it simply has no power, does a little missing but mainly feels like one big flat spot.

I had thought fuel initially, so checked the filter (bit restricted but no longer) - made no difference, i wondered about fuel pressure, i clamped the return line and it made no difference. I Can also sit and watch the fuel pouring into the motor, It runs, there is no change in fuel flow, then it dies suddenly, so i no longer think its a fueling issue.

The way it cuts out suddenly also does not seem like fuel, so now i am leaning towards the ignition module in the distributor or some other electrical problem

Just looking for some advice/experiences. These trucks are very uncommon in Australia and the fueling system especially (a carby with two injectors hung over it) is pretty foreign to me. I wondered if this might be a common issue

Pic of my truck added!

THanks

A '96 with a '88-'89 nose? On top of that you mentioned it's a TBI? Unless Australians got some special deal like Mexican GMT400's (I spy fender turnsignal indicators..) I'm thinking you've got a much earlier truck than a '96 or a custom job.. Need underhood, drivers door & RPO pics..

Anyway, with the two GMT400's I've had the only thing that's stopped either one was one of two things: The fuel pump or ignition module/dizzy. Now, since this thing is new to you, you might want to verify base timing & firing order. Who knows what the PO did to it. I also highly, highly recommend a scan tool, one that allows you to look at live data. Since they can be used on more than just your truck, it'll pay for itself. If you don't already have one.

I will take some, but you may be right, it might have been imported here in 96, thats why its complied as that date here. I normally research vehicles a bit before buying but i just loved it so i bought it heh. I have a workshop manual in the post so making do until then, im guessing timing on these would be done via a scan tool. I have three scan tools but unfortunately none will talk to it.

I think the ignition module is a good punt, in my experience they tend to work or not, but being able to watch consistant fuel entering the motor and have it suddently die still i struggle with it being fuel, though its certainly my next poart of call. I will probably get a module and a coolant temp sensor as my research has told me they are pretty critical to the system, and if that doesn't work i might look at fuel again

Tough to say without being able to look at it first hand. From that single pic it looks to be an '88 or '89. I like the glass headlight look so I've never owned one newer
If you can spring it, I recommend an old Snap-On MT2500. It's what I've got and if you get one with all the adaptors & attachments it'll work for almost anything OBD I, like your truck. Mine has paid for itself nearly 3 times over now.

These trucks aren't anything fancy, you can check the timing with a regular timing light. You first need to warm up the truck, then shut it off, unplug the EST wire (it lives under that black cover behind the A/C accumulator/drier and is a tan wire with a black stripe. Has a connector to make disconnecting it easy, kinda hard to miss unless it's been taped up. Base timing should be 0 degrees. Verify the firing order is correct first. While you're there, get a good look at the vacuum diagram on the core support and verify all lines are going where they should be.

You say the injectors are pouring fuel in, but what does the pattern look like coming from the injectors? It should be an unbroken fine spray, conical. If there are any drips or breaks in the cone, you've got leaky/dirty injectors.

Also, the dizzy in my truck failed when I went to put new plugs, wires, rotor & cap on it. But, it failed in a way where it would run but just not idle... So anything over 1000 rip 'ems was fine and dandy. However, if I let the thing try to idle on it's own, it would stumble & die. Turned out the ignition module was going bad, if I unplugged that EST wire the thing would idle just fine. New dizzy with module later and it's been going great ever since.

Don't start throwing parts at it until you've figured out what the problem is. Diagnose, not replace.

Crankshaft position sensors can cause the same issue. My 97 was throwing codes for it and eventually it got wet, which just fried it. I ended up putting a motor in, but that's just because I thought it spun a main bearing and I found a low mileage motor for cheap.

If your truck is actually an 88-89, I don't think that your scanner will work. They were still OBD1 back then, so you'd need GM specific diagnosis. Timing isn't set via the scanner on these, at least not base timing. It set by the position of the distributor gear on the crank/cam gear. You can change advance/retard slightly by turning the distributor.

Look in the driver door jamb, there should be a sticker with the month and year of manufacture.

Or grab the VIN from the plate at the driver side windshield.

Sent from my iPhone using Tapatalk

Snap a picture under the hood that will tell us whether it is TBI or Vortec. I would be extremely suspicious of fuel pressure. Also common problems that can cause similar symptoms are the ground at the thermostat housing and the coolant temperature sensor. The one in the intake.

Have a look at the VIN. The eighth digit is the engine code and the tenth digit is the model year.

Ok its a 1989 model, K on the VIN, where is got confusing is its been branded as 1996 on importing to Australia. I think i might be onto something though, i rechecked vacuum, cleaned PCV, none of it made any difference, i then checked firing order, all good. I then checked timing, it was a few degrees out so i fixed that, didn't change much again. THen i thought i would unplug a few sensors and see what made a difference and what didn't. First one was coolant temp in the manifold, i fired it up without thisplugged in and it ran, and stayed running, i took it around the block and its improved it 10 fold, smells a bit rich but it seems to run pretty decently now. So i think i will be getting a coolant temp sensor for it.

Hmm i drove it again alter in the day and it seemed to be worse again, anyway i have a coolant temp sensor coming so i will try that