Anyone knowledgeable on chevy TBIs?

Ok, finally got some time to mess with the 88 Firebird Im putting together for a British team coming over to run in BABE. Dang thing will run with some carb cleaner or gas down the throttle body but wont stay running. It will continue to run as long as I spray carb cleaner in there so obviously a fueling problem.

It does have gas (I checked that first)

The injectors are putting out fuel. At first it was just dripping out, I pulled them out and hosed them down with cleaner and now it's a halfhearted spray rather than a drip, still not as good as it could be but fuel is getting there.

A few times its even tried to fire but wont quite get there.

I did notice some fuel was coming out around the base of the throttle body so right now I have it removed so I can put a new base gasket in...maybe that will do the trick...dunno. Getting a TBI rebuild kit while I'm at it, new gaskets and seals and stuff since the thing looked pretty cruddy

Anything else I could be missing here? I put about 5 gallons of fresh gas in the tank on top of whatever might have been in there (it was last run this summer so the fuel isnt too old), and a can of seafoam. The gas coming out of the injectors is clear and clean and smells like gas so I don't think its just a matter of bad fuel.

I did fix a few vacuum leaks so I'm sure that helps but the base gasket leaking would also cause a huge vacuum leak so maybe that will do the trick.

Beyond that I'm at either injectors, fuel filter (should be done anyway) or fuel pump. The halfhearted spray concerns me that maybe there isn't enough pressure or maybe the injectors are just crudded up.

Any other tips, tricks etc anyone can think of?

I'm getting frustrated because I have the 3 required elements, fuel, spark and compression...it should run!

You need 4 things: fuel, spark, (a little) compression and timing. You appear to have spark, timing and compression. Are the injectors firing? I dunno throttle bodies. Can you look down the hole and see fuel when they fire? Is the ocmputer OK? Do you have a "noid" light? Can you put a volt meter across the injector(s) and see if they are being triggered? Does the ECu have any codes? Do you have fuel pressure? What is it?

I just went through this problem with a 1989 Nissan Sentra I just rebuilt. I had the injectors cleaned and calibrated in January but didn't get the motor back in the car until this month. In the meantime, the injector stuck shut. According to RC Engineering (a GRM advertiser), freshly serviced or old injectors can stick shut if there is too long an interval without being used. The clue is the improvement you got when you sprayed cleaner on it.

Check your fuel pressure against spec, plus read more about my problem here. See if it sounds a lot like yours. First, put the injector terminals across a 9-volt batterry (observe polarity) for just a moment to see if it clicks everytime you momentarily make the circuit. That means the injector is cycling. If no click, try taking the injector out and giving it a couple of taps. That might be enough to unstick it. If not, check out one of the companies restoring fuel injectors near you, especially if they are GRM advertisers.

A screwed up injector on a multi-port motor means at least some of the motor works. With a TBI motor, one stuck fuel injector means a whole lot o' nuthin'.

The injectors are firing, I can see them releasing fuel when I crank. At first it was "dribbling" out, once I pulled them and cleaned them they developed more of a spray pattern but not a terribly good one.

I picked up a rebuild kit today which has a new FPR so Im going to go through all that tomorrow and see if it helps any.

This is a BABE car and it's on someone elses dime so Im trying to limit the fixes to as cheap as possible especially since this one WON'T be coming back from New Orleans (anyone interested in it for challenge use, meet us in New Orleans and you can get a deal) It should be mentioned though that this is a 305 with 250,000 miles on it ;) Dunno if this thing will actually survive the trip....but I never think any of our cars will make it. I still have my doubts on the Olds I'm driving...or should I say hope to be driving since it hasnt really moved much under its own power yet ;)

You checked for good spark? Next, I guess check the timing... Also check http://www.thirdgen.org/

I get a lot of my TBI advice there.

Lugnut wrote: You checked for good spark? Next, I guess check the timing...

The fact that it will run, and continue to run (quite well) on a steady stream of carb cleaner leads me to believe all is well except the fueling.

Well, some more info into the fray.

I rebuilt the TBI and installed it...then broke the ignition (dont ask) so messed around with getting it to start without the key, fun stuff. No I didnt mess with any wiring, just pulled the lock cylinder out so we can rule that out as something I screwed up.

Back to the TBI. Through a mistake I made a discovery. I forgot to plug in the vacuum line to the MAP sensor and with that unplugged the car will start and run for about 5 seconds on its own, then the injectors cut off and it stalls.

With the vacuum line hooked up it wont even start although there is fuel coming from the injectors. It will still run on carb cleaner though.

Oh, and I checked for ECM codes and there were none

Have you checked fuel pressure? Have you used a noid light to make sure they're getting good signals? Have you considered another throttle body from a junkyard? I just got one for a megasquirt conversion I'm working on, I think the throttle body assembly (with sensors, etc.) was $25 or so at the U-Pull. At that price, it seems you would be better off than randomly rebuilding stuff trying to find the problem.

A fuel pressure gauge and noid light set are worth their weight in gold when trying to diagnose fuel delivery issues on an injected engine, IMO...super easy to check on the TBI units, too.

Bryce

Nashco wrote: Have you checked fuel pressure? Have you used a noid light to make sure they're getting good signals? Have you considered another throttle body from a junkyard? I just got one for a megasquirt conversion I'm working on, I think the throttle body assembly (with sensors, etc.) was $25 or so at the U-Pull. At that price, it seems you would be better off than randomly rebuilding stuff trying to find the problem. A fuel pressure gauge and noid light set are worth their weight in gold when trying to diagnose fuel delivery issues on an injected engine, IMO...super easy to check on the TBI units, too. Bryce

Fuel delivery (injectors) is good, fuel pressure is unknown since GM did not feel the need to put a test port on TBI cars and I haven't had time to plumb one in.

It will run and idle with the MAP and TPS disconnected unfortunately in searching some posts on thirdgen.org the only response I saw to symptoms such as those is that a fuel pump fixed it which defies logic but so does most of the system. Unfortunately in searching over there the symptoms point to many possible problems and solutions. If this was a car a gave a darn about I'd put the time effort and money into it but its a car that is making a 1500 mile drive to its death at a junkyard, nothing more. If it cant even do that then its time to part it now and move on to the next candidate. I have to get my own car prepped which will be making the return trip so my resources are required there. I have one or two new candidates lined up for the brit team and I'll mess with this one on a more leisurely schedule or just part it out which is probably its best use in life at this point.

JThw8 wrote: Fuel delivery (injectors) is good, fuel pressure is unknown since GM did not feel the need to put a test port on TBI cars and I haven't had time to plumb one in.

It doesn't have a test port because its low pressure. Just use a vacuum/fuel pressure gauge and look for 14 psi.

MAP sensors are a common failure point, but they typically don't affect startup because the ECM has a base cranking fuel map that should start the engine even without MAP input.

Look to the fuel nozzles. Notice I didn't call them injectors. They squirt liquid fuel at low pressure . Not injectors They also fail, clog, and otherwise suck from time to time.

Otherwise, stay with the fuel issue. You've already determined that you are missing the fuel since it will run with alternate fuel squirted in. Did you check TPS voltage and adjustment? How about IAT and CTS?

curtis73 wrote:

JThw8 wrote: Fuel delivery (injectors) is good, fuel pressure is unknown since GM did not feel the need to put a test port on TBI cars and I haven't had time to plumb one in.

It doesn't have a test port because its low pressure. Just use a vacuum/fuel pressure gauge and look for 14 psi. MAP sensors are a common failure point, but they typically don't affect startup because the ECM has a base cranking fuel map that should start the engine even without MAP input. Look to the fuel nozzles. Notice I didn't call them injectors. They squirt liquid fuel at low pressure . Not injectors They also fail, clog, and otherwise suck from time to time. Otherwise, stay with the fuel issue. You've already determined that you are missing the fuel since it will run with alternate fuel squirted in. Did you check TPS voltage and adjustment? How about IAT and CTS?

I can watch the nozzles while Im cranking, they are spraying fuel. With the MAP disconnected it will start and run for about 5-10 seconds then the injectors/nozzles shut off. With the MAP and TPS disconnected it started and held and idle and the nozzles were spraying. Thirdgen is saying fuel pump which just makes no sense if it runs with those disconnected. Fuel pump relay could be suspect I suppose. I guess that's the problem, too many things are suspect, and even after all this there's still a host of issues with this POS. This is one someone else's dime so if I cant get it BABE ready for say 200 or less (still needs muffler, brake pads and other bits) then its best to move on. If the car had any value post rally so they could sell and recoup a bit then maybe it would be worth it but around here 80s Firebirds and Camaros can be had for sub 1k all day long and most of them look better than this.....

curtis73 wrote: It doesn't have a test port because its low pressure. Just use a vacuum/fuel pressure gauge and look for 14 psi.

Sorry, meant to ask, where do you hook the vacuum gauge to test this with no test port?

curtis73 wrote:

JThw8 wrote: Fuel delivery (injectors) is good, fuel pressure is unknown since GM did not feel the need to put a test port on TBI cars and I haven't had time to plumb one in.

It doesn't have a test port because its low pressure. Just use a vacuum/fuel pressure gauge and look for 14 psi.

Ditto...they're easy because you can just use a regular tee on the line as you would with your vacuum/pressure gauge used on carb'd cars. If there aren't already hose-clamp type fittings to use, you can cut the hose and leave a barbed splice in your tracks when you're done. It's not so simple on newer cars, especially those with plastic fuel lines, quick disconnect fittings, etc. Of course, cars with the schrader fitting (such as older multiport GMs) are awesome for a tech, but probably not very cost effective to include that feature on every single car that will probably get used 0-2 times in the life of the car.

Bryce

JThw8 wrote:

curtis73 wrote: It doesn't have a test port because its low pressure. Just use a vacuum/fuel pressure gauge and look for 14 psi.

Sorry, meant to ask, where do you hook the vacuum gauge to test this with no test port?

See above...use a tee. If this was a car you didn't want to hack into, you could use a fancy adapter as shown below, but for this BABE-mobile the splice will be fine.

http://www.howstuffinmycarworks.com/GM_TBI_fuel_pressure.html

Bryce

Disconnecting the MAF and TPS sends the computer into "limp-home mode," a super-rich condition designed to get you home should one or more of your sensors fail. Sometimes computers are too dumb to throw a code when there's a partial sensor failure but yanking the connector looks like a total meltdown to the computer.

Sounds to me like the fuel pressure is fine since the car runs. Have you tried replugging in the MAF while keeping the TPS disconnected?

JThw8 wrote: Sorry, meant to ask, where do you hook the vacuum gauge to test this with no test port?

Just pull the fuel line off at a convenient place and put the hose from the gauge on the nipple.

The fact that you can see fuel being squirted doesn't mean that its the right amount. It could be overly rich or lean. The fact that it runs when you squirt carb cleaner in it pretty much verifies that the other systems are working right. Its gotta be fuel.

It could be a weak fuel pump, but your right it sounds counterintuitive. But if pulling the MAP sensor off makes it run, then its possible that the ECM defaults to a richer fuel map making it run long enough to run out of fuel.

Either way, the diagnostic is the same; test the fuel pressure.

injector squirting is not in direct correlation to the vehicle having proper fuel pressure.

i had a TBI van once that ran with 1psi of pressure - had no power but it ran and drive. new pump fixed it.

you can't begin to diagnose TBI without verifying that you have 9+ PSI. that is the first test every time, otherwise you're throwing parts at it.

Hmmm. It could be possible that disconnecting two sensors throws the computer into limp home mode which makes up enough of the fuel pump underdelivery to get a start and idle.

Test the fuel pressure then. I adapted my Bosch K-Jetronic fuel pressure testing rig to test 1989 Nissan Sentra fuel pressure by buying two 5/16" barbs (one male, one female on the 1/.4" NPT end. The Bosch testing unit comes with a brass "T" block) for a whole four bucks total at the local non-chain hardware. The local orange-colored chain only had 1/4 and 3/8" barbs.

That, and six inches of 5/16 FI hose and I had me a Tee-type pressure tester. It worked fine for the 45 psi pressures (without vacuum at the regulator) I was seeing so a Tee-type setup should be no problem at 14 psi.

Fuel pump on order. I tossed a new filter in tonite (for some odd reason I had one sitting in my garage....left over from...who knows) and hooked up a pressure gauge...the needle never moved so the pump sux or the gauge is bad but I tested the gauge with an air source and it read pressure.

Fuel pumps are a common issues with TBIs...

DJ

JThw8 wrote: Fuel pump on order. I tossed a new filter in tonite (for some odd reason I had one sitting in my garage....left over from...who knows) and hooked up a pressure gauge...the needle never moved so the pump sux or the gauge is bad but I tested the gauge with an air source and it read pressure.

I assume you made sure the pump is getting good power when doing your pressure test? There's a relay that controls the fuel pump, if it's not getting power to the pump you won't get any pressure. Just another thing to check before dropping the tank and swapping the pump...it'd really suck to go through that to find out you just needed to swap a relay or something like that.

Bryce

Nashco wrote:

JThw8 wrote: Fuel pump on order. I tossed a new filter in tonite (for some odd reason I had one sitting in my garage....left over from...who knows) and hooked up a pressure gauge...the needle never moved so the pump sux or the gauge is bad but I tested the gauge with an air source and it read pressure.

I assume you made sure the pump is getting good power when doing your pressure test? There's a relay that controls the fuel pump, if it's not getting power to the pump you won't get any pressure. Just another thing to check before dropping the tank and swapping the pump...it'd really suck to go through that to find out you just needed to swap a relay or something like that. Bryce

you can hear the pump run, it just doesnt make pressure.

and this is a BABE car, I'm not dropping the tank...Im goin in from the top :)