TBI 350 needs prime to start

My 88 Silverado 350 with about 400k miles wouldn't start without priming the throttle body. Initially, I thought it was out of gas. The gauge hasn't worked properly since I've owned it.

I put 7 gallons in the tank, primed the throttle body and started it up. It ran normally, but after sitting for a minute it wouldn't start again without a prime.

I can get a pretty good look at the spray pattern with an LED flashlight, and the first few times it dripped a few tiny drips of fuel from the injectors when trying to start, but now I don't see any fuel when cranking.

Now, before I go on, the fuel pump is still working. 

I can hear the pump cycling like normal, and after I prime it it runs normally. It's not starved for fuel at a higher demand level; it will pull at full throttle all the way to the shift point.

The spray pattern from the injectors looks normal when it's running.

I changed the fuel filter yesterday morning, primed it afterwards, and it started and ran normally with the prime (just like it has since this started). It then restarted fine after I finished cleaning up.

I tried it again last night after about 6 hours and it fired right up. I figured, problem solved, especially when it again started this morning like normal.

Feeling confident, I drove the truck to work, then when I got ready to come home, no start. Naturally, I had been in a rush, and didn't bring any gas to prime it. My wife had to drive the 18 miles from home to bring me a can of gas. Not a win, but to her credit, no complaints.

So, the fuel filter made a big difference but the problem still persists. I'm thinking the next step is to check out the fuel pressure regulator. 

Any comments or suggestions?

This happened to me when I was bringing back the 1990 C1500 and let it sit for a week or so. Changing the fuel filter fixed the issue.

It's a long shot, but did you put the filter in backwards?

If I remember right, fuel pressure regulators do need to hold fuel pressure for say 45 minutes to an hour but shouldn't need to hold it for 8 hours in order for the vehicle to start and run consistently.

Brett_Murphy said:

This happened to me when I was bringing back the 1990 C1500 and let it sit for a week or so. Changing the fuel filter fixed the issue.

It's a long shot, but did you put the filter in backwards?

No, I really don't think so, but I'll check it when I get back to it on Sunday. 

I'll be watching this thread for future Clem's benefit.  My '89 starts and stalls a couple times when the weather gets cold and on a first-time-in-a-few-days cold start.  I'm guessing it's a fuel issue.  Not exactly what you all are describing, but probably related.  

Coolant temp sensor wires shorted together, making it think it's really hot out?  Busted pressure regulator, although that should make it lean under power.  Corroded trigger wheel in the distributor, so it doesn't pick up ignition signal until it's running faster? (There's some logic problems with that one) 

The problem sounds awfully like when the fuel pump relay dies.  There is a backup on the oil pressure switch to power up the fuel pump as long as there is oil pressure.  In this case you just have a really extended crank (engine needs to build 7psi oil pressure for the pump to run, and then the fuel pump needs to pressurize the lines) but it will eventually run.  And, in any event, you say you can hear the fuel pump running.

I'd still want to get a fuel pressure gauge on it somehow to see what it is doing.  And a noid light on one of the injectors to see what the computer is trying to do.

Do those pumps have anti drain back/check valves? That's a common problem on some Fords that will cause extended cranking after sitting (but will usually start after cycling the pump a few times.) 

Saron81 said:

Do those pumps have anti drain back/check valves? That's a common problem on some Fords that will cause extended cranking after sitting (but will usually start after cycling the pump a few times.) 

Jeeps too. My ZJ had the drainback valve fail, so you either had to crank it for a while before it lit off, or you cycled the key a few times and it would start on about the 2nd revolution.

Thanks for the replies. Fuel pressure gauge will be next. No Schrader valve on these, so you have to put it on the fuel line at the filter.

Off to google a solution to that. 

Oh, and repeated cycling of the key and extended cranking didn't work last night. Good suggestions, though.

Well, that shoots a bunch of ideas in the foot.

Yeah, my current theory is that the in-tank filter/sock has disintegrated, clogging the filter. 

I will check the fuel pressure next.

If you have full power at redline you don't have a fuel supply issue. It sounds to me more like you have an issue with the injectors at low pulsewidths. I'd turn the key on, watch supply voltage to one of the injectors, and ground that injector (or maybe both) to see if that voltage drops considerably. Low supply voltage to the injectors would cause them to have huge latency (opening times) which at low pulsewidths basically means they wouldn't open at all. Once the engine is running it would probably fuel trim it's way back to semi-normalcy once into closed loop, and a latency issue would be a much smaller problem at high loads because it wouldn't make as large a proportional change to the total open time of the injector. Does it idle completely normally once it starts?

This is where having a scantool would be awesome, so you could see what the long term fuel trim (or the GM equivalent was called - block learn?) was, and so forth.

A TPS that is reading really high might be putting it into clear-flood mode, too, although that should also set a code because it is rare for a TPS to go that far out of whack without sending signal voltage over 4.5v, which should be an instant circuit-fault code.

In reply to Vigo :

Yes, it idles and runs normally. As in, like it's got 15,000 miles on it, instead of 30 times that. 

Your theory makes sense, and I'd rather work it up some more before I start throwing parts at it. I'm still going to replace the fuel pump at some point, so I can get the gauge working properly. It moves, but in a range from dead empty when there is ten gallons or less in the tank, to 1/3 when the tank is full.

You will need one of these to check the fuel pressure. It's easier than splicing the line and you can leave it in while you diagnose the problem.

sorry for the large image.  My bet is you need a new fuel pump, TBI will run just fine with extremely low pressure.  Spec is around 13 psi IIRC, mine ran with as little as 9.  The only symptom i had was extended cranking and some stalling in warm weather.  You should be able to find the adapter on ebay or amazon.

The computer doesn't need anything as far as inputs to open the injectors during 'priming'. So, if the injectors aren't opening it's probably a latency thing as mentioned. However, if they ARE opening and there's no fuel to release because the fuel pump isn't running, it probably has something to do with the fuel pump relay. The fuel pump can be powered two ways on these silly things. One is by the ECM turning on the fuel pump relay, and the other is by power going through the oil pressure switch. You would only get that power after cranking long enough to make oil pressure, and that power comes out of a fusible link and goes through some splices so it could be wiring problems as well. 

Simplest thing to do without having any special tools is make the fuel pump run by bypassing the relay or powering it through the diagnostic connector as some obd1 chevys (maybe all?) had a terminal on the diagnostic connector that would power the fuel pump.  Once the fuel pump is jumped to be running, if the engine cranks right up you know your initial injector pulse is there. If it doesnt you have to go look for it and if you can't visually verify it you'd be looking at doing some testing at the injector connectors with a multimeter, test light, or noid light. 

Update: changing the fuel filter was enough for me, I took it into the shop to get it diagnosed and repaired.

The new fuel filter wasn't clogged, the pump puts out 11 PSI which is right in the middle for a TBI system.

I'm getting referred tomorrow to someone to chase down an electrical issue. 

Just to make it more difficult, while I had to prime it again to drive it to the shop, it started right up the next morning while it was there. We'll see what it does tomorrow when I pick it up.

Not just electrical, but intermittent as well.

In reply to Floating Doc :

those are always fun, I had my mustang doing a crank no-start intermittantly years ago and chased it all over. Turned out to just be a bad intermittant wiring connection to the fuel pump. I guess vibration from it running would keep it powered enough or something. Was finally found after much expense and blind alleys including a new fuel pump and paying a pro to remove an aftermarket alarm that a PO had installed. 

GOOD LUCK!

Thanks, I'm going to need some luck. My regular tech (who is lobbying for a 383 swap) referred me to another shop.

Of course, it started right up when I picked it up this morning after sitting for two days.

Price out a 383 swap, and price out a junkyard LS swap.  If it's even cash-on-par, you're ahead with the LS.

"Hmm, there's a possible electrical problem.

I know, let's introduce a whole bunch of variables without actually fixing what is wrong first!"

SkinnyG said:

Price out a 383 swap, and price out a junkyard LS swap.  If it's even cash-on-par, you're ahead with the LS.

I think a junkyard LS with a 4L80E would work out better than a 383 in front of my 700R4. 

However, I don't have time or motivation for a swap. I think buying a newer 2500 GM will be the long term plan. I just need to put It off as long as possible.

I bought two cars in the last year, currently need to rebuild the bank account (we're on a cash only budget).

Update: as in, nothing to update.

The truck failed to start only once the entire time it was at the other shop, and then when they had gathered the testing equipment it fired right up.

After trying it multiple times over several weeks, they called me to pick it up. It started up that morning, I drove it home, and it hasn't started since. 

They told me to call the next time it failed to start so they could come check it in place. I'll give them a call on Monday morning.

So, once it starts, it will stay running without a problem but if it sits for a while, it won't re-start?

Sounds like a failing but not yet dead fuel pump. Sometimes they will pump no problem, other times they won't spin up and you won't get fuel.

Try to start the truck, if it won't start, get a block of wood or your boot and give the tank really good thump. If the truck starts after that, you have a bad pump that is intermittent but not dead.